Automatically adjusted STAIR_ARROWs now stop at the top thread

Previously they were ending at the end of the stair object and problem was that the end of the object could be at the slab underneath the stair.

Example - https://i.imgur.com/Y43v0GM.png
Before - https://i.imgur.com/Ze9DoK2.png

src/blenderbim/blenderbim/tool/drawing.py

@@ -97,13 +97,18 @@ class Drawing(blenderbim.core.tool.Drawing):
                 arrow.parent = stair
 
                 # place the arrow
+                # NOTE: may not work correctly in EDIT mode
                 bbox = tool.Blender.get_object_bounding_box(stair)
+                float_is_zero = lambda f: 0.0001 >= f >= -0.0001
                 arrow.location = Vector((bbox["min_x"], (bbox["max_y"] - bbox["min_y"]) / 2, bbox["max_z"]))
+                last_step_x = max(v.co.x for v in stair.data.vertices if float_is_zero(v.co.z - bbox["max_z"]))
+
                 arrow.data.splines[0].points[0].co = Vector((0, 0, 0, 1))
-                arrow.data.splines[0].points[1].co = Vector((bbox["max_x"], 0, 0, 1))
+                arrow.data.splines[0].points[1].co = Vector((last_step_x, 0, 0, 1))
 
                 if not cls.get_assigned_product(arrow_element):
                     tool.Ifc.run("drawing.assign_product", relating_product=stair_entity, related_object=arrow_element)
+                pass
 
     @classmethod
     def create_camera(cls, name, matrix):