Enable user selection of backend

[pbartholomew08]

Description: Currently the backend is selected based on the detected compiler: nvfort -> GPU/CUDAFortran; other -> CPU/OMP. If we add a GPU/OMP backend this should be supported by nvfort (which could also support CPU/OMP too), therefore the user should be able to select the backend independently of the compiler.

Requirements:

1. Invalid backend/compiler combinations should be rejected (primarily only nvfort supports GPU/CUDAFortran)
2. The defaults should be preserved (nvfort->GPU/CUDAFortran, other->CPU/OMP)