group together the MIxS 'sections' that are currently implemented as parent slots?

[turbomam]

see

• Find elements whose names contain white-space #1875 (comment)

coud group with a new, is_a parent slot, or a in_subset, or a slot_group!

• we have been following a trajectory of removing subsets from nmdc-schema
• as of today, our only slot_groups are technically undefined

[turbomam]

alternatively:

• delete the MIxS 'sections' that are currently implemented as parent slots? #2698