snglcoinc:  simplify some cruft
snglcoinc:  add FIXME to CoincSynthesizer

- comment from Jolien that came up in the review.
snglcoinc:  can rely on scipy providing speed of light

- lal does, too, and this is py*LAL* but we need it as a float, and so
  getting it from scipy is one less line of code