Add Butterworth filter option to FFTSmooth

[mantid-builder]

Original Reporter: @DennisMikkelson

Original Owner: @DennisMikkelson

A Butterworth filter option can be easily added to FFTSmooth so that noisy data can be smoothed without introducing the "ringing" artifacts caused by an abrupt frequency cutoff as is done by the "Zeroing" option. This will require two positive integer parameters, a cutoff control parameter, n, and the order of the filter.

[mantid-builder]

This issue was originally trac ticket 1477