PSD plotting CWFollowup script

A CWFollowup script that uses lalapps_ComputePSD and gnuplot to plot the region around (in frequency space) a candidate. The script overplots the location of the candidate. For binary sources (which are all that’s supported for now), it also calculates the size of the “wings” caused by Doppler modulation and plots those. The plot range is (slightly more than) the width of the wings, or 150 bins, whichever is more. (For presentation purposes it seems helpful to have a reasonable number of bins in the plot).

Still to do: proper test case, isolated sources, some documentation