Get rid of MarginalModel

[ricardoV94]

Now that we have model transforms established in PyMC, I think we can do this via a model transform that eagerly places the MarginalRV is in the graph. Everything else should work the same, except unmarginalize will have to know how to look for them, because they are not saved as a model property. Shouldn't be terrible.

This will make allowing user to specify different types of marginalization / parameters for them as in #353