XLAL(Multi)AddToFakeSFTCatalog(): build a 'fake' SFT catalog

- Contains only detector and timestamp information, which may
  then be extracted by functions which operate on SFT catalogs
- 'Fake' SFT catalogs are sorted using the same ordering as
  XLALSFTdataFind(), i.e. by GPS time using compareSFTdesc()
- XLALAddToFakeSFTCatalog() creates a catalog or adds to an
  existing one; takes a single detector name and timestamps
- XLALMultiAddToFakeSFTCatalog() is a multi- wrapper
XLALCWSignalCoveringBand(): compute covering frequency band for CW signals

- covering band includes spindown evolution between two GPS times,
  maximum Doppler modulation due to detector motion, and (for binary
  signals) maximum Doppler modulation due to orbital motion