Added DISTANCE subtype to BIM Tool Extrusion Depth and Length

In BIM Tool it looks like this right now - https://i.imgur.com/JWVPssg.png (note the units)
Previously it was displaying value like this - https://i.imgur.com/244aYTN.png

The benefit is that you can specify other units in that kind of property and it will automatically convert to current project's unit. 

For example if you have millimeters as project's unit you can still type "10m" instead of "5000" and it will automatically convert it to 5000mm. 
Or type in inches like 5" 11'.

src/blenderbim/blenderbim/bim/module/model/prop.py

@@ -86,7 +86,7 @@ class BIMModelProperties(PropertyGroup):
     )
     occurrence_name_function: bpy.props.StringProperty(name="Occurrence Name Function")
     getter_enum = {"ifc_class": get_ifc_class, "relating_type": get_relating_type_id}
-    extrusion_depth: bpy.props.FloatProperty(default=42.0)
+    extrusion_depth: bpy.props.FloatProperty(default=42.0, subtype="DISTANCE")
     cardinal_point: bpy.props.EnumProperty(
         items=(
             # TODO: complain to buildingSMART
@@ -113,7 +113,7 @@ class BIMModelProperties(PropertyGroup):
         name="Cardinal Point",
         default="5",
     )
-    length: bpy.props.FloatProperty(default=42.0)
+    length: bpy.props.FloatProperty(default=42.0, subtype="DISTANCE")
     openings: bpy.props.CollectionProperty(type=ObjProperty)
     x: bpy.props.FloatProperty(name="X", default=0.5)
     y: bpy.props.FloatProperty(name="Y", default=0.5)