OpenMP parallelization of 3D Classic

This pull request contains OpenMP parallelization of the 3D Clawpack Classic code, at the level of step3/step3ds.  This is joint work with with Kenny Roche (@rochekj).  The bulk of the computation should be parallelized, but I haven't done weak scaling tests yet.  Strong scaling is okay, but not great; on the 100^3 and 200^3 grids I used with the tests/acoustics3d and tests/acoustics3d_dimsplit examples, I observed roughly 60% parallel efficiency or better (with higher efficiency for fewer threads, as one would expect).  Because of the way the code is parallelized, results should be bit-for-bit reproducible independent of the number of threads used.  I encourage rigorous testing.