A lot of code simplification

A lot of simplifications allowed having a much more straightforward code. Thanks to Andrew Xie (@IAmAndrewX) for a very productive summer trimming down code, replacing the old MTLPP with Swift, and making a new class arrangement for the different GPU backends. Now BabelViscoFDTD is based completely on PyOpenCL and PyCUDA for the FDTD viscoelastic solver. For Metal, the Swift-based wrapper does the interfacing. The old C extension is still around just for the OpenMP backend.