CFS Resampling: refactoring step1

- integrated old-API ComputeFstat_RS() function into new-API ComputeFstat_Resamp()
- completely XLALified
- passes testCFSv2_resamp.sh and testGCT.sh
- refs #535
ComputeFstat: consistently return '2F' from any '*ComputeFstat*' function

- adapted ComputeFstatFromFaFb() and XLALComputeFstatFromAtoms(), while
  XLALComputeFstat() already followed this convention.
- refs #1362