Contact-Homotopy Updates (Cache results of Tribol calls, etc).

[thartland]

This PR adds

• initial caching of results from Tribol contact mechanics computations. That is, repeated calls to the same function at the same point will not lead to additional contact mechanics computations.
• update ContinuationSolvers in order to avoid compilation errors when users compile smith with ContinuationSolvers enabled and no direct solver (STRUMPACK, MUMPS, CPARDISO) in the mfem build. Thanks Chris for bringing this issue to my attention.
• complete caching of results from Tribol mechanics computations. That is, when the gap constraints are evaluated internally Tribol also computes gap constraint Jacobians.

Regarding item (3) -- there is now a structure in the Constraint class to communicate that we are requesting a "fresh_derivative" but not at a "new point". By "new point" we mean has the Constraint class "seen" this point in it's most recent computation (perhaps as just a constraint evaluation), so that a derived Constraint class can cache the Jacobian computations from the constraint evaluation and just provide the cached Jacobian when a "fresh derivative" is requested at an "old point".