DOC/ENH : get/set_size_inches

Added links between `get_size_inches` and `set_size_inches`

Added an `np.array` call to `get_size_inches` to return a copy of
the size, instead of a reference to the internal np.array.

Closes #2303
DOC : minor tweaks to set/get_size_inches

 - better numpydoc style
 - added inch <-> cm conversion factor
DOC : transposed incorrect section title
Merge pull request #2836 from tacaswell/set_size_doc

DOC/ENH : get/set_size_inches