BUG: several nanops fail when axis==0 for 1-dimensional nan arrays

[toddrjen]

Several of the functions in nanops, including nanmean, nanmedian, and anything using _get_counts will crash when using a 1-dimensional all-nan array when the axis argument is set to 0.

>>> from pandas.core import nanops
>>> import bottleneck
>>> import numpy as np
>>> nanops._USE_BOTTLENECK = False
>>> 
>>> val = np.tile(np.nan, (5,))
>>> 
>>> bottleneck.nanmean(val, axis=0)
nan
>>> nanops.nanmean(val, axis=0)
TypeError: 'numpy.float64' object does not support item assignment

[jreback]

closed by #7354