NormalizeSFTRngMed: allow assuming a fixed noise PSD value

- XLALNormalizeSFT() and related functions now take an assumeSqrtSX scalar
  which if >0 gives the noise PSD value to assume.
- XLALNormalizeMultiSFTVect() now takes an assumeSqrtSX MultiNoiseFloor
  array for assumed per-detector noise PSD values.
- Refs #1181

Signed-off-by: Reinhard Prix <reinhard.prix@ligo.org>