Update term uniqueness checker to check universal uniqueness

[matuskalas]

Atm, the check is missing uniqueness including both label and a synonym. See bug edamontology/edamontology#741.

And there is no check of uniqueness between "sub-ontologies". Let's add a warning for now, to see what the situation is.

Plus please add related_term into the checks, just like any other synonym. (An even better solution would be inferring that it is a special case of a synonym, but I assume that might be possible only with an OWL/RDF reasoner(?))

Related note, not necessarily a part of this issue: I'm considering making file extensions searchable in BioPortal etc., by also defining them as a special case of a synonym. Not strictly related to that, but it'd be great for us to see whether the file extensions are unique or overlap somewhere (possibly also with a main label of a different format). But of course unlike other terms, these can overlap with the main label of the given format (e.g. label PDF and fileext pdf).

Another note: I assume the check if case-insensitive, i.e. the same term with just different capitalisation will fail the uniqueness check, right?

• Fail if the same term is used as the main label and as a synonym anywhere within the same "sub-ontology".
• Output a warning if the same term is used in multiple sub-ontologies, irrespective of whether it is the main label or a synonym.
• Include related_term into the check, fail if not unique.