WIP: More housekeeping

- Rename FFT classes so that they follow PEP8 naming conventions. The following classes have been renamed:
    - fft -> FFT
    - fft_fftw -> FFTW
    - fft_gfft -> GFFT
- Add docstrings for new arguments and for the old ones missing one
- Add axes keyword to rg_space.calc_transform()
- More PEP8 fixes in nifty_fft
- GFFT's transform method now uses paradict instead of para
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