Added docs for new linearization methods

Added doc page for linearization methods in physics/mechanics.
Added api for linearize
Merge branch 'new_docs' into MechanicsDocumentation
Fixes to msubs to make it work in Python2

Also fixed docstrings to pass doctests.
Documentation updates for new functionality

Added documentation for linearization methods, and all assorted changes
that occurred along with them.
Finishing touches on mechanics doc updates

- Updates to docstrings to render well in sphinx docs
- Added prose walkthrough of example sections
- Spelling and grammar fixes