allow >1 chromosome in recomb collector

[petrelharp]

I think it just works if we pass in locus_position as cumulative and length as total length and have loci on bot ends of all chroms.

[petrelharp]

seems to! shld document.

[petrelharp]

see https://github.com/petrelharp/local_pca/blob/master/sims/local/threeway-local-fixed-s-sim.py#L142