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
TST : added test to exercise transform kwarg to `add_label_near`

This exercises with input in both display and data units.
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