Revert "XLALSFTdataFind: change convention for SFT being in a GPS interval [start,end)"

- This reverts commit d480378aac1696f37d616f8c3adb77c81fe2edc8.
- Convention based only on start time is actually preferable, as it
  ensures that, for a contiguous segment list, SFTs will always fall
  in exactly one segment, whereas otherwise an SFT that overlaps the
  boundary between two segments would be missed
- The previous Change in convention results in an inconsistency between
  XLALSFTdataFind() and segment reading in LALApps codes, which breaks
  existing search setups
- refs #1286
XLALParseMultiNoiseFloor(): special handling of single-value sqrtSX input

- if sqrtSX->length=1 < numDetectors: use that single number for all detectors X,
  which allows for more user-friendly input of simple cases
- passes 'make check'
- refs #1181
PredictFStat: new input argument --assumeSqrtSX, deprecated --SignalOnly

- can specify non-unity per-detector noise floors with --assumeSqrtSX
  (doesn't load the SFTs into memory in that case, only uses their catalog)
- extended testPredictFStat.sh to cover the new option
- passes 'make distcheck' on jenkins
- refs #1181