Update Axes3D.tricontour for custom triangulations

An example has also been added to illustrate the additional flexibility
possible with providing a user-defined triangulation.
Make Axes3D.contourf take arbitrary triangulations
Add Axes3D.contourf example
Update changelog/whats_new for 3D tricontour plots
Typos
Add EXPERIMENTAL note to Axes3D.tricontour[f] docs
Remove she-bang line.
Use consistent imports
Remove unnecessary code comments.
Remove unnecessary calls to `show`
Rmove unused variable
Split up hist examples into two sections.

The original example was trying to do too much in one place.
Remove use of `setp` in favor of keyword args.
Replace multiple figures with subplots
Change bin edges to make the example clearer.
Add titles to clarify intent
Simplify example
Split demo that tried to do too much
Remove unnecessary second histogram
Remove unnecessary parameters