Improvements and cleanup to the routines fft_forward and fft_backward routines
Renamed and overloaded the different flavours of fft_forward and fft_backward. Removed two wrapper functions in nfft that seemed useless and renamed some of the routines for greater consistency.
- I have checked that my code follows the Octopus coding standards
- I have added tests for all the new features added in this request.
Closes #297 (closed)