gstlal_inspiral_injection_snr: made number of parallel processes a command line argument. Default = 1

gstlal_inspiral_pipe now uses reference_psd.cache for gstlal_median_of_psds

split up for computing injection snrs, then merged back together before
being merged with the associated injection LLOID files

committed with permission)

gstlal_inspiral_pipe: Remove FIXME that was addressed by commit 416627b7

- in the FIR-whitener code path

- only add a pre-whitener reblock element in the non-FIR whitener code path

- check that psd_fft_length is an integer

- confirm the channel list matches the instruments in the time slide vectors
- confirm the reference PSDs, if given, match the instruments in the time slides

- in gstlal_inspiral and gstlal_fake_frames automatically enable PSD tracking when a reference PSD is not given instead of reporting it as an error
- in mkwhitened_multirate_src() require track_psd to be set if psd is None
- flatten initialization of whitener's psd-mode property

gstlal_inspiral_pipe: Set svd job priority to 99. Should make all svd jobs submitted before insiral jobs