rethink is_a dh_interface requirement for including class in menu system?

[turbomam]

I think that LinkML schema classes aren't included in the template menu unless they is_a: dh_interface

is_a is single-valued, so reserving it for this purpose eliminates its usefulness for making real hierarchical statements about classes. We could switch to

mixins:

• dh_interface

but I think @cmungall may have something else in mind

cc @pkalita-lbl