pass in grids in step separate from model initiation

[sbenthall]

Subproblem of #914 see also #1171

Currently grids over state spaces are computed when a model is initialized based parameters handed to the model during construction.

We would like this to be a two step process, so the same model can be given different grids.

Includes a choice -- should the the data type of the grid be a numpy array? or a more expressive type like a new Grid class?