Aggregating over arrays
This _is_ frowned upon behaviour (storing arrays inside DataFrames) but is there a reason for this raise? 

Deleting the raising lines seems to only break tests to check that they're raising... 

```
df = pd.DataFrame([[1,np.array([10,20,30])],
               [1,np.array([40,50,60])], 
               [2,np.array([20,30,40])],], columns=['category','arraydata'])
g = df.groupby('category')
```

```
g.agg(sum)
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
<ipython-input-34-527a2010b455> in <module>()
----> 1 g.agg(sum)

/Users/andy/pandas/pandas/core/groupby.py in agg(self, func, *args, **kwargs)
    337     @Appender(_agg_doc)
    338     def agg(self, func, *args, **kwargs):
--> 339         return self.aggregate(func, *args, **kwargs)
    340
    341     def _iterate_slices(self):

/Users/andy/pandas/pandas/core/groupby.py in aggregate(self, arg, *args, **kwargs)
   1740             cyfunc = _intercept_cython(arg)
   1741             if cyfunc and not args and not kwargs:
-> 1742                 return getattr(self, cyfunc)()
   1743
   1744             if self.grouper.nkeys > 1:

/Users/andy/pandas/pandas/core/groupby.py in f(self)
     62             raise SpecificationError(str(e))
     63         except Exception:
---> 64             result = self.aggregate(lambda x: npfunc(x, axis=self.axis))
     65             if _convert:
     66                 result = result.convert_objects()

/Users/andy/pandas/pandas/core/groupby.py in aggregate(self, arg, *args, **kwargs)
   1745                 return self._python_agg_general(arg, *args, **kwargs)
   1746             else:
-> 1747                 result = self._aggregate_generic(arg, *args, **kwargs)
   1748
   1749         if not self.as_index:

/Users/andy/pandas/pandas/core/groupby.py in _aggregate_generic(self, func, *args, **kwargs)
   1803                     result[name] = self._try_cast(func(data, *args, **kwargs),data)
   1804             except Exception:
-> 1805                 return self._aggregate_item_by_item(func, *args, **kwargs)
   1806         else:
   1807             for name in self.indices:

/Users/andy/pandas/pandas/core/groupby.py in _aggregate_item_by_item(self, func, *args, **kwargs)
   1828                 colg = SeriesGroupBy(obj[item], selection=item,
   1829                                      grouper=self.grouper)
-> 1830                 result[item] = colg.aggregate(func, *args, **kwargs)
   1831             except ValueError:
   1832                 cannot_agg.append(item)

/Users/andy/pandas/pandas/core/groupby.py in aggregate(self, func_or_funcs, *args, **kwargs)
   1425                 return self._python_agg_general(func_or_funcs, *args, **kwargs)
   1426             except Exception:
-> 1427                 result = self._aggregate_named(func_or_funcs, *args, **kwargs)
   1428
   1429             index = Index(sorted(result), name=self.grouper.names[0])

/Users/andy/pandas/pandas/core/groupby.py in _aggregate_named(self, func, *args, **kwargs)
   1509             output = func(group, *args, **kwargs)
   1510             if isinstance(output, np.ndarray):
-> 1511                 raise Exception('Must produce aggregated value')
   1512             result[name] = self._try_cast(output, group)
   1513

Exception: Must produce aggregated value
```

http://stackoverflow.com/questions/16975318/pandas-aggregate-when-column-contains-numpy-arrays
