Implemented RangeIndexGenerator, and a CLI command to call it.

RangeIndexGenerator produces the RangeIndexEntry values that can be written into a range index file.
It collapses consecutive ReferenceRegions, corresponding to the genotype rows, into a single ReferenceRegion
splitting only on boundaries between sites that are mroe than a threshold distance apart (10kb by default).

IndexFlatGenotype is a CLI command used to invoke the RangeIndexGenerator.