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