Added weakkeydict
WeakRef dictionary by itself is essentially the same as no cache. Added
a fixed length deque to keep track of some keys
most of the key,value pairs we want to cache are not weak-referenceable,
get around this limitation by using both weakkey and weakvalue dicts
fixed typo in return statement
Implemented Python 3 functools.lru_cache as a class decorator compatible
with py2 and 3
Removed locks and some if statements to improve performance
add variable size to lru_cache through environment variable SYMPY_CACHE_SIZE
cache_debug now appropriately handles SYMPY_CACHE_SIZE
Various items listed as autoclass which should be autofunction
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
Added code minus tests
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 basic vector tests
Merge branch 'master' of https://github.com/sympy/sympy into basicvector