Generate relational schema

[r-peschke]

As agreed let us merge this PR to the feature-branch

[jsangmeister]

Small typos or clarifications. The concept itself looks good. I'm missing a list of required attributes for reach relation type, e.g.:

• 1:1 must always contain a reference on one side and a sql on the other
• similar rules for 1:n and m:n

(just an example because I don't know the actual rules yet)

This can then be validated if we want via the to attributes, which is another reason why I would like to keep them.

[r-peschke]

Small typos or clarifications. The concept itself looks good. I'm missing a list of required attributes for reach relation type, e.g.:

* 1:1 must always contain a `reference` on one side and a `sql` on the other

* similar rules for 1:n and m:n

(just an example because I don't know the actual rules yet)

This can then be validated if we want via the to attributes, which is another reason why I would like to keep them.

This is documented in models.yml as list of implemented from-to-rules.