New column `Source_Material`

[stschiff]

We will introduce a new column called Source_Material (before Source_Tissue), which will be strictly a choice-field with options petrous, bone, tooth, soft, sediment, other. Here, bone means any other bone material than petrous.

It will be a list-field and the number of items must match the number in Source_Tissue.

We would clarify in the manuals that Source_Tissue should be used as a more free field to specify further the specifics of the sampled material, including cases where it's not actually a tissue (like sediments). That's somewhat of a naming inconsistency, but we can live with that.

[nevrome]

I'm closing issues that are already implemented in #93.