Move BC-parsing helper to BaseCase
This commit moves the helper function that process expansion/boundary conditions ("FOURIER" -> PERIODIC, etc) to BaseCase.cpp, from Options. This change should be invisible to most case files, which habitually include BaseCase for helper functions as-is, but it allows Options to once again be independent of NSIntegrator. This improves modularity for other uses of the code, such as a hypothetical SPINSoff (get it? it's a pun) that would like to do some spectral voodoo with an option file but doesn't need the full Navier-Stokes machinery.