REF: _AXIS_TO_AXIS_NUMBER to simplify axis access
This adds a dict called ``_AXIS_TO_AXIS_NUMBER `` to NDFrame/DataFrame where the keys are the allowed parameter values for the ``axis`` parameter in various ndframe methods and the dict values are the related axis number. This makes getting to the correct axis more straight forward, see for example the new ``_get_axis_number``, and makes adding type hints to the ``axis`` parameter easier.