Added a chromosome generation script that uses msprime instead of ms

[Jazpy]

script should be run in the following way:

python3 msprime_chromosomes.py [-h] --gens GENS --bases BASES [--split [SPLIT]]

Where:

• GENS represents the age of of the ancient sample measured in generations elapsed.
• BASES is the length of the chromosomes that will be simulated.
• SPLIT can specify a mass migration that separated the ancient and contaminant populations. This is also measured in generations elapsed.

The script is intended to emulate ms2chromosomes.py output, but it can be easily modified to incorporate more samples and other demographic events.

[grenaud]

thank you to Jazeps Medina Tretmanis for this contribution.