tools for multiple chromosome

[petrelharp]

Once SLiM moves to the 'one ts per chromosome' model, we'll need some new tools. The main consideration here is that the nodes/indivs/pops tables need to agree between all the chromosomes' tree sequences.

For starting a SLiM simulation with msprime, I see two options:

1. simulate the whole genome in msprime then chop it up into a bunch of separate tree sequences
2. simulate each chromosome independently then union together the nodes tables, effectively saying that the sets of ancestors are disjoint between chromosomes