Add a validator to warn user if they use at least one but not all of the params in an 'infix' predicate definition

[ym-han]

Add a validator to warn user if they use at least one but not all of the params in an 'infix' predicate definition, e.g.

GIVEN (some_person: Person
another_person: Person)
DECIDE some_person has helped
IF True
(This is purely stylistic, but could still be confusing to a reader; hence the need for a validator)