separate model and solver attributes

[chriscoey]

only 3 overlap, and they don't really make sense (per discussion with @mlubin). so separate these attributes.

also from discussion with @mlubin: define an interface for asking a solver whether a group of "types" of constraints is supported together. and also a way to ask the solver, given an existing model that has already been solved, what types of constraints can be added to it. EDIT moved to #8

there is confusion on my part on what a model is and what a solver is. I think perhaps the terminology could be revisited to make it clearer.

[chriscoey]

@mlubin suggested Instance or SolverInstance instead of Model. maybe Solver is fine as-is

[chriscoey]

in #7, I renamed model to instance, and also separated instance and solver attributes