Turn on ROBOT's multiple_equivalent_class_definitions check

[hoodcm]

Part of #5.

Check	Data checked	Why
multiple_equivalent_class_definitions	Logical axioms	A class has at most one logical definition.

ROBOT's standard check, at its default level (ERROR). Nothing on main fails it, so no data changes. All tests pass.

[jclerman]

I would not merge this. Multiple logical definitions on a single class can actually be quite informative, e.g.

left knee
== knee and (part_of some left leg)
== knee and (part of some left side of body)

The argument for this rule in the ROBOT docs is vague (to me), and I've found considerable utility in the kinds of constructions it prohibits.