`n_fft`, `frame_length` and `hop_length` should be constrained to sensible values

there are currently no constraints on the above varialbes (used for deriving audio indexes in `process_files.py`).

These variables interact in ways that might make data output non-valid in non-obvious ways.

The user should at least be warned if they have selected bad values, but the script should probably just not run in that case.