Schema: make controlled concept fields a structure with label and ID

[smrgeoinfo]

per tech meeting 2023-03-31, resolve question of what value should be used in controlled concept fields (has_context_category, has_material_category, has_specimen_category) -- a text string or a URI.

Solution-- make into a structure with mandatory URI and optional label. URI is required to be in a vocabulary that has as its top concept a value from the iSamples vocabulary for that sample classification dimension.

[smrgeoinfo]

implement in 137_controlled_concept_object branch

[smrgeoinfo]

resolved with #140, close