Should we make nucleation probability into fields with a global index?

[fractalsbyx]

This would save a lot of overhead that would otherwise be needed on the backend like specialized dependencies and avoid trouble with dofhandlers. Similarly to how we have an is_postprocess attribute, we can have an is_nucleation_probability attribute. Fields that nucleate can have an attribute that is an index to the probability field, making it much easier to have multiple fields share a nucleation probability without performance loss or weird UI.