aggregation ops for object-dtype are missing

[fujiisoup]

This issue arises in #1837 comment, where we need to make a summation of object-dtype array, such as

xr.DataArray(np.array([True, True, False, np.nan], dtype=object), dims='x').sum('x', skipna=True)

Currently, it raises NotImplementedError.

pandas support this by having their own nan-aggregation methods.