Fix ambiguity over convention for when a GPS time is 'within' a range

- XLALCWGPSinRange() defines the official CW convention, which matches
  that used by XLALSFTdataFind() and the GCT code's segment reading
- use this function in SFTfileIO.c, HierarchSearchGCT.c and ComputePSD.c
  to decide whether an SFT start time is within a given range
- rename SFTConstraints fields to less ambiguous minStartTime and maxStartTime,
  since the 'end time' of an SFT is never used in the constraints
- replace endTime/maxEndTime with maxStartTime throughout, including
  command-line arguments, variable names, and comments
- refs #1286

Signed-off-by: Reinhard Prix <reinhard.prix@ligo.org>