ComputeFstat_Demod: dismantled old-API remnants now hidden behind new-API

- removed Fcomponents type, subsumed ComputeFBuffer into FstatInput_Demod
- cleaned up buffering
- improved efficiency for multi-frequency binary search
- removed Demod() usage from Resamp(), plus temporary fixes to Resamp
  to avoid too slow performance in testHS
- refs #543
ComputeFstat_Demod.c: pure comment and formatting cleanup

- removed references to 'old' vs 'new API' now that old-API has been removed
- re-ordered definitions in slightly more logical order
- changed function-defintion style back to 'repr' ;)
- no content changes other than: use '0' as COMPLEX16-initializer instead of INIT_MEM
- refs #543