PyCharm warnings on numeric operations

[max-sixty]

Currently PyCharm warns if a numeric operation is performed on an xarray object. We could suppress these by:

• Adding __add__ = NotImplemented onto the BaseDataObject (and all the other ops from CMP_BINARY_OPS & NUM_BINARY_OPS), so IDEs know about them
• Adding a set of skeletons here: https://github.com/JetBrains/python-skeletons. I haven't tried this, and the readme looks out of date. But it does seem to work for numpy objects
• Wait for IDEs to be able to recognize the monkey patching that is done here (not my preference)

[shoyer]

I'd be happy with the first option, although I'd rather we use a function that actually gives a sensible error message, i.e.,

def _not_implemented(*args):
    return NotImplemented
__add__ = _not_implemented

This does seem to be a point in favor of doing something more explicit rather than injecting numeric operations.