Fstat: replaced array of FstatFaFb={Fa,Fb} struct by separate arrays Fa[],Fb[]

- more direct, easier to work with downstream
- refs #1434
Fstat: promoted ComputeFstatFromFaFb() to full API function

- can be useful also outside of this module,
- used in TransientCW_utils instead of manual F-stat computation
- refs #1434
Fstat codes: remove redundant precision in Fstat, Fa,Fb and A,B,C,E,D

- consistently use Fstat=REAL4, FaFb=COMPLEX8, A,B,C,E,D=REAL4
  everywhere in 'modern' Fstat codes
- refs #1434
LALXMLTest: increased tolerances slightly more to avoid random test failures

- tested in ~1000 trials to be safe,
- *and* fixed random seed to avoid 'random' surprises
- fixes #1448