SCUBA2: Update FTS-2 band pass boundaries

Recent Spectral Filter Profile (SFP) work has allowed for greater precision
on the desired band pass boundaries for FTS-2 spectra
SCUBA2: Add new optional fNyquist override parm

The default Nyquist frequency used during interferogram initialization
is derived from the STEPTIME and SCANVEL and is geared towards
a value of 50.0 when the SCANVEL is at its nominal rate of 4.2 mm/sec.