Create a child workplane on a vertex (#480)

* add parent face detection to workplane()

* Update cq.py

* add nonetype check and tests

* fixed test formatting

* mypy fix attempt

Co-authored-by: Adam Urbańczyk <adam-urbanczyk@users.noreply.github.com>

tests/test_cadquery.py

@@ -3157,6 +3157,22 @@ class TestCadQuery(BaseTest):
             result.faces(">Z").val().Center().toTuple(), (-3, 0, 12), 9
         )
 
+    def testWorkplaneOrientationOnVertex(self):
+
+        # create a 10 unit sized cube on the XY plane
+        parent = Workplane("XY").rect(10.0, 10.0).extrude(10)
+
+        # assert that the direction tuples reflect accordingly
+        assert parent.plane.xDir.toTuple() == approx((1.0, 0.0, 0.0))
+        assert parent.plane.zDir.toTuple() == approx((0.0, 0.0, 1.0))
+
+        # select the <XZ vertex on the <Y face and create a new workplane.
+        child = parent.faces("<Y").vertices("<XZ").workplane()
+
+        # assert that the direction tuples reflect the new workplane on the <Y face
+        assert child.plane.xDir.toTuple() == approx((1.0, 0.0, -0.0))
+        assert child.plane.zDir.toTuple() == approx((0.0, -1.0, -0.0))
+
     def testTagSelectors(self):
 
         result0 = Workplane("XY").box(1, 1, 1).tag("box").sphere(1)