Verify algebraic properties of monoids in monoid_valid predicate

[jennalwise]

• verify that the binary operator of a monoid is associative
• verify that the identity element of a monoid is in fact an identity element of the binary operator and monoid domain
• validate any other algebraic properties of a monoid

[jennalwise]

This extends to other methods using monoids:

• [ ] GrB_Monoid_new, GB_Monoid_new