When adding new components, foreign keys have to be updated as well

[jnnr]

When extending the components in oemoflex, one must not forget to add them to the foreign keys as well.

Problem: When passing component_attrs_update to the method EnergyDatapackage.setup_default(), one would need to update the foreign keys as well, which is not done atm. One solution would be to generate the list of foreign keys automatically from the component definitions.

[jnnr]

To be more precise: component_attrs.yml contains info on foreign keys, which setup_default uses to generate foreign key references for all the regions or links. But there is a separate foreign_keys.yml which contains the same information. It would be better to use a single source of truth for this information.