Add helpful suggestion to parse_gate when called with non-basic gate.

This also makes the error message consistent with the one in `phase_block_optimize`.

See #196 for context.

pyzx/optimize.py

@@ -512,9 +512,9 @@ def parse_gate(self, g: Gate) -> None:
             else: # Only the control has a hadamard gate in front of it
                 self.add_hadamard(c)
                 self.add_cnot(g)
-        
+
         else:
-            raise TypeError("Unknown gate {}".format(str(g)))
+            raise TypeError("Unknown gate {}. Maybe simplify the gates with circuit.to_basic_gates()?".format(str(g)))