add a feature to mask high rec regions

[mufernando]

I think it would be helpful to have a feature to mask super high recombination rate regions (relative to the specified mutation rate). these windows just take too long in SINGER.

I know @nspope has artificially tuning down rec rates when needed. this could also be a feature (maybe in place of the hard filter).

[nspope]

good idea, I think the operations are scale around a desired mean then clamp (or maybe the other way round). it looks like I already did the re-scale on a branch, so i'll add the clamp and push it shortly.