genome array compacted

[molpopgen]

Attempt to:

1. Append new genomes to current contiguous array
2. Then, move all genomes associated w/new births to the
   beginning of the Vec, over-writing the dead genomes.

The problem is that the borrow checker hates this type of code.
We are both taking slices to view parents and appending to the end.

Getting this to work in general will require moving away from code in src/common.rs
and trying to avoid storing borrows with lifetimes.
(However, there is a simplistic hack for non-overlapping generations, which is simply to move
all the new data from a separate instance of genomes.)