FFt filtering to remove ripple

ASAP 2.4 and onwards offers the ability to flag data in the fft'ed spectrum and either return the selected region for use with scantable.lag_flag or apply it directly to the spectrum.

Example

from asap.lagflagger import lagplotter
s = scantable('myrippleddata.rpf')
lp = lagplotter(s)
lagregions = lp.flag()
#do flagging in the plot window.
lagstart = lagregions[0][0]
lagend = lagregions[0][1]
s.lag_flag(lagstart, lagend, unit='')