lal_transferfunction:  new property tf-on-nongap allowing TFs to be calculated anytime there is nongap data

gstlal-calibration:  New element lal_calibcorr, to compute residual errors in the calibration based on comb injections and make FIR filters that apply compensation to each component of the calibration.  It builds, but it may not work properly yet.

line subtraction ringing test: fixed TF plotting.  lal_transferfunction:  working to add df and f0 properties

FIRtools.py:  added specialized fft function rfft_comb(), useful for measuring linearly-spaced combs