Move CSTWmarket into ConsAggShocksModel

[mnwhite]

As is, cstwMPC defines the relevant Market subclass for doing consumption-saving problems with aggregate shocks (to find a dynamic general equilibrium). This class should be moved to ConsAggShocksModel (and renamed). Further, the relevant simulation (etc) methods should be moved to a ConsumerType extension in this module as well. cstwMPC.py should import these classes, and should only contain functions that could only plausibly be used for cstwMPC.

A necessary subtask of this is to rework the slope_prev and intercept_prev functionality, as storing these in a different module/namespace is clunky and weird. The dynamics calculator should be a class method, not a function; this will allow us to store the previous parameters inside the class.

[mnwhite]

Done in recent commit to comments-and-cleanup branch.