Validate that a digital object only has one Non-Aggregator Project

[cgalarza]

We need to ensure that a digital object only has one non-aggregator project. A digital object can have any number of aggregator projects. We could do this by
a) have a two separate fields, aggregator project and project. For each we would have to validate that the correct project type is in the right field. Ensure that only one project is listed.
b) Keep the project field and at validation time check that only one non-aggregator project is listed.
c) something else?

My preference (Carla) is option b because this makes it easier if we were to expand to other types of project, but I'll leave this decision to the implementor.