Issue title: Problem with the total_energy() function in the IAWE dataset.
 Issue body: Usage:

```
elec.total_energy()
```

Stack trace:

```
Calculating total_energy for ElecMeterID(instance=2, building=1, dataset='iAWE') ...   ---------------------------------------------------------------------------
HDF5ExtError                              Traceback (most recent call last)
<ipython-input-164-22e855a674ac> in <module>()
----> 1 elec.total_energy()

/Users/rishi/Documents/Master_folder/IIITD/5th_semester/Independent_Project/nilmtk/nilmtk/metergroup.pyc in total_energy(self, **load_kwargs)
    810 
    811         meter_energies = self._collect_stats_on_all_meters(
--> 812             load_kwargs, 'total_energy', full_results)
    813 
    814         if meter_energies:

/Users/rishi/Documents/Master_folder/IIITD/5th_semester/Independent_Project/nilmtk/nilmtk/metergroup.pyc in _collect_stats_on_all_meters(self, load_kwargs, func, full_results)
    826             print_on_line("\rCalculating", func, "for", meter.identifier, "...   ")
    827             single_stat = getattr(meter, func)(full_results=full_results,
--> 828                                                **load_kwargs)
    829             collected_stats.append(single_stat)
    830             if (full_results and len(self.meters) > 1 and

/Users/rishi/Documents/Master_folder/IIITD/5th_semester/Independent_Project/nilmtk/nilmtk/elecmeter.pyc in total_energy(self, **loader_kwargs)
    586         nodes = [Clip, TotalEnergy]
    587         return self._get_stat_from_cache_or_compute(
--> 588             nodes, TotalEnergy.results_class(), loader_kwargs)
    589 
    590     def dropout_rate(self, ignore_gaps=True, **loader_kwargs):

/Users/rishi/Documents/Master_folder/IIITD/5th_semester/Independent_Project/nilmtk/nilmtk/elecmeter.pyc in _get_stat_from_cache_or_compute(self, nodes, results_obj, loader_kwargs)
    714             stat_for_store = computed_result.results.export_to_cache()
    715             try:
--> 716                 self.store.append(key_for_cached_stat, stat_for_store)
    717             except ValueError:
    718                 # the old table probably had different columns

/Users/rishi/Documents/Master_folder/IIITD/5th_semester/Independent_Project/nilmtk/nilmtk/docinherit.pyc in f(*args, **kwargs)
     44         @wraps(self.mthd, assigned=('__name__','__module__'))
     45         def f(*args, **kwargs):
---> 46             return self.mthd(obj, *args, **kwargs)
     47 
     48         return self.use_parent_doc(f, overridden)

/Users/rishi/Documents/Master_folder/IIITD/5th_semester/Independent_Project/nilmtk/nilmtk/datastore/hdfdatastore.pyc in append(self, key, value)
    154         data in the table, so be careful.
    155         """
--> 156         self.store.append(key=key, value=value)
    157         self.store.flush()
    158 

/Users/rishi/anaconda/lib/python2.7/site-packages/pandas-0.14.1_345_ge226bac-py2.7-macosx-10.5-x86_64.egg/pandas/io/pytables.pyc in append(self, key, value, format, append, columns, dropna, **kwargs)
    912         kwargs = self._validate_format(format, kwargs)
    913         self._write_to_group(key, value, append=append, dropna=dropna,
--> 914                              **kwargs)
    915 
    916     def append_to_multiple(self, d, value, selector, data_columns=None,

/Users/rishi/anaconda/lib/python2.7/site-packages/pandas-0.14.1_345_ge226bac-py2.7-macosx-10.5-x86_64.egg/pandas/io/pytables.pyc in _write_to_group(self, key, value, format, index, append, complib, encoding, **kwargs)
   1246                 group = self.get_node(new_path)
   1247                 if group is None:
-> 1248                     group = self._handle.create_group(path, p)
   1249                 path = new_path
   1250 

/Users/rishi/anaconda/lib/python2.7/site-packages/tables/file.pyc in create_group(self, where, name, title, filters, createparents)
    957         _checkfilters(filters)
    958         return Group(parentnode, name,
--> 959                      title=title, new=True, filters=filters)
    960 
    961     createGroup = previous_api(create_group)

/Users/rishi/anaconda/lib/python2.7/site-packages/tables/group.pyc in __init__(self, parentnode, name, title, new, filters, _log)
    239 
    240         # Finally, set up this object as a node.
--> 241         super(Group, self).__init__(parentnode, name, _log)
    242 
    243     def _g_post_init_hook(self):

/Users/rishi/anaconda/lib/python2.7/site-packages/tables/node.pyc in __init__(self, parentnode, name, _log)
    282             # We do *not* rely on ``__del__()`` doing it later,
    283             # since it might never be called anyway.
--> 284             self._f_close()
    285             raise
    286 

/Users/rishi/anaconda/lib/python2.7/site-packages/tables/group.pyc in _f_close(self)
    913         # be loaded during the closing process; thus this node must be
    914         # open until the very end.
--> 915         self._g_close()
    916 
    917     def _g_remove(self, recursive=False, force=False):

/Users/rishi/anaconda/lib/python2.7/site-packages/tables/group.pyc in _g_close(self)
    880             # hdf5extension operations:
    881             #   Close HDF5 group.
--> 882             self._g_close_group()
    883 
    884         # Close myself as a node.

/Users/rishi/anaconda/lib/python2.7/site-packages/tables/hdf5extension.so in tables.hdf5extension.Group._g_close_group (tables/hdf5extension.c:9912)()

HDF5ExtError: HDF5 error back trace

  File "H5G.c", line 811, in H5Gclose
    not a group

End of HDF5 error back trace

Problems closing the Group meter2

```

Because it has problems in closing meter 2, this affects the pairwise_correlation() function, causing it to never end.

 Comments: 
 Comment 0: Please be more specific
On 29 Dec 2014 20:06, "Rishi Baijal" crystalsandoval@example.org wrote:

> —
> Reply to this email directly or view it on GitHub
> https://github.com/nilmtk/nilmtk/issues/329.

 Comment 1: Works for me. Closing.
{'source': 'GithubIssues', 'repo_name_with_owner': 'nilmtk/nilmtk', 'issue_id': 53067667, 'repo_stars': 691.0}