Proposal: Rename "contig" to "chromosome" for hail 0.2

This has always confused me and others.

I was leaning the other way, toward using contig more consistently since these really are contiguous sequences of DNA bases, not necessarily any of the 22 human chromosomes. On build 38, I think we’ll have thousands of ALT contigs, MT, (Epstein-Barr and other decoys?) in the reference.