BUG: Wishart node's __init__ handled plates incorrectly
ENH: Implement broadcasting to "Kalman smoother" and some other funcs

Function block_banded_solve was moved from the utils.utils module to
utils.linalg module. It now handles broadcasting properly. This function
provides the same result as Kalman filter + RTS smoother but is more
general and handles broadcasting.

chol_solve now accepts matrix inputs, that is, if your system is A*B=Y
instead of A*b=y. Broadcasting is handled properly in this case too.
Just provide `matrix=True` keyword argument.

utils.linalg.mmdot implements matrix-matrix dot product with
broadcasting
ENH: Implement drifting Gaussian Markov chain

Implemented the drifting Gaussian Markov chain model. Refactored code
such that there is a general abstract class which can be used for
sub-classing variants Gaussian Markov chains.