Create an abstract Constraints class to support different types of constraints via inheritance

[faisal-bhuiyan]

Task
Create an abstract interface for different types of constraints supported in OpenTurbine.

Why
To treat the constraints uniformly as member of the base class, rather than duplicating code to acknowledge the different types.

Done means

• Refactoring of the constraints classes to establish an inheritance hierarchy
• Unit tests exist for all derived classes

[deslaughter]

I gave this a bit more thought and I don't think the inheritance based approach will work well with the Kokkos parallelism. If we could define a vector of Constraint type, it would be a good solution, but inevitably we'll need all the data to generate Phi and B in the GPU and we won't want to copy that in at every iteration. I do think this may be a great opportunity to define a View of Constraint structs that we can do a parallel_for over. Let's get together and come up with a list of constraint types that the struct will need to implement.