Boundary condition refactor

[rrsettgast]

The initial conditions were recently refactored as part of the SinglePhase_TPFA solver development. The Boundary conditions should follow suit. Also, the Boundary Condition files could use some cleanup.

[rrsettgast]

@klevzoff I think that we can greatly simplify the bc code by calling the lambda in the set loop. One of us should do this and hack out a bunch of code.

[herve-gross]

Would be great to assign boundary conditions to sets of volumes/faces/edges/nodes (respective examples: aquifer/faults/wells/fracture tips).

[rrsettgast]

@herve-gross Did you close this accidentally?

[herve-gross]

Yes, my mistake! This definitely needs to be put on the queue of important features.