Updated with core developer feedback: fixed for Tonne and added tests

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

@@ -209,6 +209,7 @@ si_conversions = {
     "pound": 0.454,
     "ton UK": 1016.0469088,
     "ton US": 907.18474,
+    "tonne": 1000.0,
     "lbf": 4.4482216153,
     "kip": 4448.2216153,
     "psi": 6894.7572932,
@@ -253,6 +254,7 @@ imperial_types = {
     "pound": "MASSUNIT",
     "ton UK": "MASSUNIT",
     "ton US": "MASSUNIT",
+    "tonne": "MASSUNIT",
     "lbf": "FORCEUNIT",
     "kip": "FORCEUNIT",
     "psi": "PRESSUREUNIT",
@@ -323,6 +325,7 @@ unit_symbols = {
     "pound": "lb",
     "ton UK": "ton",
     "ton US": "ton",
+    "tonne": "t",
     "lbf": "lbf",
     "kip": "kip",
     "psi": "psi",