df.groupby() does not allow to use name of index at level 0 when not using a MultiIndex
``` python
In [24]: df = pd.DataFrame({
   ....:         'exp' : ['A']*3 + ['B']*3,
   ....:         'var1' : range(6),
   ....:         })

In [25]: df = df.set_index(['exp'])

In [26]: df.index.names
Out[27]: ['exp']


In [28]: df
Out[28]:
     var1
exp
A       0
A       1
A       2
B       3
B       4
B       5

In [29]: try:
   ....:             df.groupby(level='exp').size()
   ....: except ValueError as e:
   ....:             print('index can not be accessed by name')
   ....:             print(e)
   ....:
index can not be accessed by name
level > 0 only valid with MultiIndex
# at this point I would argue that the level 'exp' is level 0,
# and therefore this should work.

In [29]: df1 = df.groupby(level=0).size()

In [30]: df1
Out[30]:
exp
A      3
B      3
dtype: int64
```
