suzerain: #2979 implicit, hybrid machinery

Prepares operator_hybrid_isothermal to modify the right hand sides
produced by operator_nonlinear whenever the grid is one sided.
Does so by:
 1) Making operator_hybrid_isothermal a nonlinear operator, too.
 2) Wrapping a shared #N and invoking it on apply_operator
 3) Making that wrapped version generate NRBC matrices in member storage
    once per RK step.
 4) Initializing those matrices to be NAN as a defensive member.
 5) Hooking the accumulate calls to selectively use these matrices.

Include having Giles methods return PG_VL_S_RY not ImPG_VL_S_RY.

Includes bugfix re: premature return from compute_giles_matrices_XXX.
Removes unused compute_giles_matrices_lower from treatment_nonreflecting.

Prepares perfect gas driver to try out the logic.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43830 0bd85f0b-c151-43b9-a1e9-27231104551f