Edits to PiFM Translator, Added a Bandpass Filter class to processing/fft

Created by: rajgiriUW

The PiFM Translator seemed to fail for the newest version of software that includes a Power Spectrum, and also the description for the conventional spectra is now 7 lines instead of 3 so that part needed to change. Checked on a few old datasets from older versions of software and it seems to work.

Added a Bandpass Filter class to processing/fft that allows an optional finite impulse response (or boxcar). A harmonic filter of num_harm = 1 is essentially the same thing, but the fir option is new.