Add msprime examples (or a separate notebook?)

[hyanwong]

At the moment the tskit.ipynb file doesn't have anything about simulation. It might be nice just to have a trivial msprime example in the tskit.ipynb notebook. This will require adding msprime 1.4.0 when it is released.

We could also have a separate "simulation" notebook, , and then we can grab the demographic model in the loaded tree sequence, using the example in the ARG workbook 2:

cmd, parameters = msprime.provenance.parse_provenance(arg.provenance(0), arg)
...