Added unit tests covering predicate signature matching.

tests/data/test-cases/skeleton.pddl

@@ -0,0 +1,36 @@
+(define
+	(domain skeleton)
+	(:requirements :typing)
+	(:types
+		a1 a2 a3 - object
+		b1 - a1
+		b2 - a2
+		b3 - a3
+		bx - a1
+		bx - a2
+		bx - a3
+		c1 - b1
+		c2 - b2
+		c3 - b3
+		cx - c1
+		cx - c2
+		cx - c3
+	)
+	(:predicates
+		(p1 ?x - a1 ?y - a2 ?z - a3)
+		(p2 ?x - a1 ?y - b2 ?z - c3)
+		(p3 ?x - (either a1 a2) ?y ?z - (either b1 b3))
+	)
+	(:constants
+		ca1 - a1
+		ca2 - a2
+		ca3 - a3
+		cb1 - b1
+		cb2 - b2
+		cb3 - b3
+		cbx - bx
+		cc1 - c1
+		cc2 - c2
+		cc3 - c3
+		ccx - cx
+	)