Review the role of GCIs with complex LHS in base extraction

We have recently finally defined a recommendation for shaping base files: https://github.com/OBOAcademy/obook/pull/387

The one thing that is still not addressed is what to do about GCIs with complex RHS, which are used plentiful in Uberon and PATO.

Currently, these are _not_ included in the base files. We need to review that decision to be sure that this is the right way to go!

We could potentially use some kind of tagging approach to force certain axioms into the base, even if they are not covered by the general process.

cc @dosumis @cmungall @balhoff

Start by reading the base-file recommendation in PR #387, then examine how complex-RHS GCIs from Uberon and PATO are currently treated. Determine whether excluding them is correct or whether tagging is needed; the work is done when the policy and resulting extraction behavior are resolved and validated.

Written by the indexing model from the issue text.