Introduce instance-level requirements into the schemas

[Raphael-Gazzotti]

This issue is linked to #513.

We may want to add requirements at the instance-level of our schemas.

e. g., a ResearchProduct requires to have a contribution/contributor with specific ContributorType/ContributionType. It makes sense to enforce that a Software product always includes at least one contribution/contributor whose contribution type is development/developer. Currently, our schema formalism does not allow specifying that if _type is Software then at least one contribution must have ContributionType="development"/ContributorType="developer". In other words, we can require the presence of a contribution/contributor field, but not enforce the kind of contribution/contributor.

A major point to address is that schemas would no longer be entirely independent from the instances.