Reordered 3d Classic code

Reordered 3D Classic code, based as much as possible on 3D PyClaw reordered Fortran for commonality's sake.  Adds dynamic allocation, removes maxmx etc.  This pull request also includes a variable-coefficient acoustics example, which uses the solver from the Riemann repo and the updated data.py from my clawutil pull request.  I've tested it against the equivalent Clawpack 4.3 problem, and the final output files are identical.