gstlal-inspiral: move SNR PDF machinery into inspiral_extrinsics

- gather all code related to initialization maintenance and retrieval of
  SNR PDFs into SNRPDF class in inspiral_extrinscs

- remove the code used to populate the cache of PDFs from the two
  create_prior_diststats tools and from calc_rank_pdfs into a new,
  stand-alone tool gstlal_inspiral_make_snr_pdf