Data model and Tables API section

[jeromekelleher]

I think we should have a section that describes the data model and (briefly) the Tables API. A diagram of the various entities would be helpful here?

[benjeffery]

I made this UML a little while back:

Here's the VPD file (editable at https://online.visual-paradigm.com):
tskit.vpd.zip

[jeromekelleher]

Looks like a good starting point to me.

I wonder if we should leave the migration table out of this for the purposes of the paper - we don't do anything with them in the library, and arguably they are redundant now we have the full ARG support in msprime (which I think stores the same information, via nodes and edges).