add support for SPECFEM2D acoustic domain

[bch0w]

Relevant Issues: #151 #182 #185

SPECFEM2D in an acoustic domain has some specific filenaming schema for kernels and acoustic/pressure waveforms that do not play nice with the current generalized Solver configuration. One example is that the kernels have an additional _acoustic tag that is not recognized by some of the wildcard statements required for file manipulation.

Likely the most productive way to solve this is to create a 2D acoustic example problem that will help address the differences with the acoustic problem and provide a longer term check on this functionality.