Add fonts to families

Script MT to cursive and Humor Sans to fantasy
Update fonts_demo

Print all text within figure and remove missing cursive text
Improve fonts_demo_kw
Merge pull request #3801 from jenshnielsen/font_cleanup

DOC : Fonts demos improvements
Change covariance to be positive-semidefinite.

Minimal changes to the plot but fixes a warning and more realistic data
Use kwarg for loc in findobj_demo.py
BUG : fix #3805

Enusure that norm.v{min,max} are not `None` before expanding
them to ensure that 0 < vmax-vmin.
Merge pull request #3807 from jenshnielsen/examples_warnings

A couple of simple to fix warnings in the examples
TST : added ColorbarBase smoketest
Merge pull request #3808 from tacaswell/colorbar_norm_bug

BUG : fix #3805
extend #if to include both CLONGDOUBLE related definitions
Merge pull request #3810 from jbmohler/extend_longdouble_conditional

extend #if to include both CLONGDOUBLE related definitions
allow selecting the backend by setting the environment variable MPL_BACKEND
added documentation on backend selection methods
minor fix for the documentation on backend selection methods (either of the methods -> any of the methods)
DOC : edits to copy of usage_faq.rst

Made how to set backend an ordered list
copy all array_view members in copy constructor
Fix image orientation on Mac OSX backend. #3806
Use XINCREF in copy constructor and subview constructor, since arr may be NULL
Added a warning about global use of MPLBACKEND or use of -dbackend