Need marker traits for metadata.

[molpopgen]

Currently, a single trait defines metadata. This leads to a potential problem for external APIs writing "export to tskit" functions. One could send the wrong metadata to the wrong table.

Since all metadata need the same API, then a simple set of marker traits will suffice to distinguish Node from Individual metadata, etc..

[molpopgen]

simple set of marker traits will suffice to distinguish Node from Individual metadata, etc..

This is harder than it sounds.