DOC : improved `extent` of `imshow` doc

closes #990
slight edits to docs as suggested by @mdboom
BUGFIX:  This change fixes #2475, where contour labels added manually
produce ugly spikes in the contours.  The problem was that screen
coordinates were being passed as data coordinates when the contour was
split to add room for the label.

Incorporate my fix and miili's #2806 change
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
TST : added test to exercise transform kwarg to `add_label_near`

This exercises with input in both display and data units.
DOC : minor tweaks to set/get_size_inches

 - better numpydoc style
 - added inch <-> cm conversion factor
Use 'y#' instead of 's#' in write_rgba of the agg backend. Fixes #1891.
DOC : transposed incorrect section title
Merge pull request #2836 from tacaswell/set_size_doc

DOC/ENH : get/set_size_inches
Merge pull request #2739 from tacaswell/extent_doc

DOC : improved `extent` of `imshow` doc
Removed 'ipython -pylab' references
Removed references to notebook
Merge pull request #3007 from cgardn/issue3005

#3005 - Removed 'ipython -pylab' references
Prevent mpl_PyFile_Dup from failing when given BytesIO or similar
Merge pull request #2898 from bytbox/v1.3.x

Fix animation errors
DOC : improved add_label_near

 - added some in-line comments to explain what is going on
DOC : cleaned up/clarified docstring
Merge pull request #2843 from tacaswell/contour_fix

BUGFIX:  This change fixes #2475, where contour labels added manually