SWIG: fix reverse operators for LIGOTimeGPS

- Reverse operators were not working because first operator argument was
  required to be a LIGOTimeGPS; this was because the specialised input
  typemaps for LIGOTimeGPS, which allow conversion from double, were defined
  *after* the definition of LIGOTimeGPS, and were therefore not applied to
  the SWIG wrapping code, including operators, defined in the %extend block.
- Split lalswig_ligotimegps.i into two SWIG headers SWIGLIGOTimeGPS{Pre|Post}.i
  which are included before|after definition of LIGOTimeGPS in LALDatatypes.h.
  SWIGLIGOTimeGPSPre has typemaps, SWIGLIGOTimeGPSPost contains %extend block.
- Test script check_lalswig.py now checks LIGOTimeGPS reverse operators work.
- Operator functions can't have keyword arguments, so remove this feature in
  swiglal_python.i (for some reason it wasn't being applied before).
- Refs #1295