REF: Keep original copies of endog, exog.
FIX: Fixed graphs in newer Pandas / Matplotlib
BUILD: Reset default sphinx html build options
ENH: Improved handling of Cython matrix dimensions
- Clarified operational vs in-memory dimensions
- Fixed the behavior of changing dimensions (e.g
  when the filter_method changes)
- Cython functions now use dimension defined in
  the appropriate object
- Moved all functionality relating to Statespace
  initialization to the ?Statespace classes
  (rather than having it spread out in the
  ?KalmanFilter and ?KalmanSmoother classes also)
- Clarified when the Representation class needs to
  create new Cython objects versus when it needs
  to re-initialize the Cython arrays to new
  dimensions.
REF: Consistent results obj in simulation smoother