Parametric object gizmo scale follows scene unit (#1715)

* Add missing units in si-conversions

* Fix error when using Miles or Thou-inch units

* Parametric gizmo scales with scene units

* use "thou" instead of "thousandth of an inch"

src/ifcopenshell-python/ifcopenshell/util/unit.py

@@ -154,18 +154,22 @@ named_dimensions = {
 }
 
 si_conversions = {
+    "thou": 0.0000254,
     "inch": 0.0254,
     "foot": 0.3048,
     "yard": 0.914,
     "mile": 1609,
+    "square thou": 6.4516e-10,
     "square inch": 0.0006452,
     "square foot": 0.09290304,
     "square yard": 0.83612736,
     "acre": 4046.86,
     "square mile": 2588881,
+    "cubic thou": 1.6387064e-14,
     "cubic inch": 0.00001639,
     "cubic foot": 0.02831684671168849,
     "cubic yard": 0.7636,
+    "cubic mile": 4165509529,
     "litre": 0.001,
     "fluid ounce UK": 0.0000284130625,
     "fluid ounce US": 0.00002957353,