ComputeFstat: new setup API

- Rename ...FstatInputData... structs/functions to ...FstatInput...
- Create the FstatInput struct using XLALCreateFstatInput_Demod() or
  _Resamp() as desired, passing any algorithm-specific parameters here.
- Then call general function XLALSetupFstatInput(), which loads SFTs from
  a catalog or generates them in memory, optionally adding injected signals.
- XLALComputeFstat() no longer adds +4 to F-statistics in "signal-only" case;
  use XLALAdd4ToFstatResults() if this normalisation is still desired.
- Ported LALApps codes to new API, but no injection functionality yet.
- Refs #1002