VariantData.from_tree_sequence

[hyanwong]

In many cases we want to infer from an existing tree sequence. At the moment we use SampleData.from_tree_sequence(). Following on from VariantData.from_arrays() in #924 , I think it would also be useful to have the from_tree_sequence() method. This is in addition to sgkit-dev/bio2zarr#232 (which would provide essentially the same functionality).

I think this would be useful for writing the unit tests too.