Added two POSEIDON.yml fields to specify a data licence for a package

[nevrome]

I think it was @martynamolak who proposed this initially and I think it is a good feature to have. I tried to make it pretty flexible and, of course, fully optional.

The british spelling of "licence" nauseates me a bit, but I guess we should stick to it for consistency.

[stschiff]

OK, that's definitely already fine as suggested here. Just some points that I'm thinking about:

In the YAML, I think I may actually prefer a hierarchical encoding, so:

- licence:
  - short_name: CC-BY-4.0
  - file: LICENCE.TXT

which I think we can encode through our levels in the POSEIDON_yml_fields.tsv, right?

I also would suggest to be more explicit that the file is more optional than the short name. I think it would lower the barrier to use licenses if one can just plug in a short name without having to find the text on the internet and copy it here.

[nevrome]

Good point! I'll implement that