Fix creation of equivalent class axioms from template

[beckyjackson]

• docs/ have been added/updated
• tests have been added/updated
• mvn verify says all tests pass
• mvn site says all JavaDocs correct
• CHANGELOG.md has been updated

Currently, template creates an intersection for all equivalent class assertions, even if there is only one equivalent expression. Intersections should only be used for 2+ expressions. See: obi-ontology/obi#1500

[beckyjackson]

Note the difference between using EC and using C with an "equivalent" class type, from the docs:

• EC %: the class will be asserted to be an equivalent class of the intersection of all EC class expressions in a row
• equivalent: values of all C columns will be taken as an intersection and asserted to be an equivalent class

This is maintained in this PR (hence the new legacy-template.owl which keeps the intersection), but a single equivalent expression will still never be an intersection. This is to maintain backwards compatibility so users don't see any unexpected changes in their axioms. Essentially, using EC will create a single equivalent axiom whereas using C will create equivalencies between multiple expressions.

This shows up the same in Protege, but the axioms look a bit different in OWL.