Explicitly Manage Boundary Conditions

[pulsipher]

Currently, we define boundary conditions as constraints, but the model doesn't explicitly track/identify these. We should explicitly track these to enhance error checking and better support transformation methods.

One possibility would be to tag them:

@constraint(model, [constr_expr], BoundaryCondition)

And/or we can automatically detect them by interrogating each constraint made.

With these constraints identified, we can potentially work to avoid boundary condition conflicts. This has some connection to #166 and #167. This should be helpful with PDE transformation methods.