Use CHMO terms instead of free strings for electrodes

[cthoyt]

metadata-schema/linkml/system/electrode.yaml

Line 45 in a3658b7

- value: "working electrode"

yesterday, we talked about using CHMO values throughout echemdb. one place where this is possible is to use CHMO terms that are children of "electrode" (http://purl.obolibrary.org/obo/CHMO_0002344) to describe the electrode types

this would also mean making a few new term requests to CHMO's GitHub repository: https://github.com/rsc-ontologies/rsc-cmo that are children of "electrode" to cover all use cases