Modified latex printing of Lambda to use \mapsto
Modified tests so working with \mapsto version of Lambda
Removed  that I forgot about in test_latex.py
Modified tests to include parentheses around whole Lambda expression
Corrected tests for Lambda
Started work on Linearizer Class

Implemented routine for constraints on position and velocity.
Unsure if it will work if either are missing, most likely will need
to add control flow for this. The API is also complicated, and will
be simplified as more code is ported over.

Also added one test for the new linearize routine. Based off of
the example in Luke and Gilbert's paper. As more functionality is
added to the linearization routine, the work done in this test
will get smaller. Still need to add more tests from the examples
I worked out last week.
Lots of work on linearization methods

- Added `to_linearizer` method to KanesMethod class. This returns
an instance of Linearizer with all information needed for the class.

- Made 1 meaninful test with the rolling disc test case. The other
two tests aren't finished yet.

- _subs_keep_derivs was written. This substitutes in values for symbols,
but keeps the derivatives of the from being substituted out.
Assorted fixes in Linearizer Class

- Forgot to multiply A and B by P^T when outputting just the
A and B matrices. Fixed.

- Added an eigenvalues test in the rollingdisc test

- Formatting and code quality fixes
Line3D class
Added support for no f_c/f_v in Linearizer

The Linearizer class now supports linearization for systems with
 - Both configuration and motion constraints
 - Only configuration constraints
 - Only velocity constraints
 - No constraints

Added a test for no constraints. Still need tests for just f_c and
just f_v. Added rough beginnings of `linearize` class method for
KanesMethod. The interface still needs to be decided on.
Generalized Linearization Routine

The following changes were made to the Linearizer Class:

1.) Now accepts if q, u, or any of the f_# generalized terms
are empty matrices. Linearization proceeds as it should.

2.) Broke the linearization process into a couple "private"
class methods. The ones done so far only need to be run once
upon initialization. This should save calculation time if the
user intends to do multiple linearizations about different
operating points.
Added new methods along the Ray class
Changes to Linearizer and Related tests

- All tests now are valid and pass
- Improved doc strings, still needs more work
- Modified tests for KanesMethod to use the new functionality

Now have the same functionality as before with linearizing
KanesMethod objects, but code is more generalized and easily
extended.
Added the remaining methods
Add medium [skip ci]
Code cleanup, pep8 fixes, docstrings edits

- Code cleanup with flake8 for pep8 fixes in new code
- Edits to docstrings
- Fixed doc tests
Add feature to initialize medium with refractive index. [skip ci]
Import constants like permittivity of free space, speed of light in vacuum and magnetic permeability from sympy.physics.units rather than using Symbols for them.
Add tests for medium
Add args test for Medium
Added tests