Add `fftw` to the modules for most computers

[sbryngelson]

After #620 is merged, we will default to looking for the system's fftw rather than building it ourselves. We will build it ourselves if fftw is not found on the system.

We can find fftw by adding it to the modules on the computers in . ./mfc.sh load.

Most computers we have modules for in ./mfc.sh load also have fftw available, which is good (it will speed-up builds). We will want to ensure this fftw is compatible with the compilers we are loading. So, it will require testing that it works before merging in the module change).

We will want to module load fftw even on GPU builds because post_process will want to use it.

This will be a gradual change as we update modules on various systems, though it will speed up the ./mfc.sh build process.