Plasmid selection simulation

Simulation of selection of plasmid mutant selection in the face of multi-copy plasmids.

Assumptions

1. Some "ideal" copy number N (configurable). If n < N, all plasmids are duplicated. If n > N, all plasmids have an N/n chance of being retained
2. Some "ideal" population size P (configurable). If p < P at the end of a generation, no action is taken. If P > p at the end of a generation, each cell has a P/p chance of surviving.
3. Cells survive if they have even 1 copy of the selection gene. Possible enhancement would be to have chance of survival proportional to the copy number (up to some maximum), but this is not implemented

Simulation

1. All cells exactly duplicate their plasmids (only keeping track of + or -).
2. Plasmids are randomly assigned to daughter cells.
3. Plasmid numbers are normalized according to assumption 1
4. Any cells lacking at least 1 positive plasmid are removed (see assumption 3)
5. If population size is too large, cells are randomly culled (see assumption 2)