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.