Fix #5964. Only get the unit assignment from the project to prevent ambiguity.

I really don't know why I didn't do this before.

src/bonsai/bonsai/bim/import_ifc.py

@@ -893,9 +893,9 @@ class IfcImporter:
         tool.Loader.set_unit_scale(self.unit_scale)
 
     def set_units(self):
-        if not (units := self.file.by_type("IfcUnitAssignment")):
+        if not (assignment := self.file.by_type("IfcProject")[0].UnitsInContext):
             return  # Geometry is optional in IFC
-        for unit in units[0].Units:
+        for unit in assignment.Units:
             if unit.is_a("IfcNamedUnit") and unit.UnitType == "LENGTHUNIT":
                 if unit.is_a("IfcSIUnit"):
                     bpy.context.scene.unit_settings.system = "METRIC"

src/bonsai/bonsai/tool/unit.py

@@ -116,9 +116,9 @@ class Unit(bonsai.core.tool.Unit):
         for unit_class in ["IfcDerivedUnit", "IfcMonetaryUnit", "IfcNamedUnit"]:
             units += tool.Ifc.get().by_type(unit_class)
 
-        assigned_units = tool.Ifc.get().by_type("IfcUnitAssignment")
-        if assigned_units:
-            assigned_units = assigned_units[0].Units
+        assigned_units = []
+        if assignment := tool.Ifc.get().by_type("IfcProject")[0].UnitsInContext:
+            assigned_units = assignment.Units
 
         for unit in units:
             name = ""
@@ -162,8 +162,7 @@ class Unit(bonsai.core.tool.Unit):
 
     @classmethod
     def get_project_currency_unit(cls) -> Union[ifcopenshell.entity_instance, None]:
-        unit_assignments = tool.Ifc.get().by_type("IfcUnitAssignment")
-        for assignment in unit_assignments:
+        if assignment := tool.Ifc.get().by_type("IfcProject")[0].UnitsInContext:
             for unit in assignment.Units:
                 if unit.is_a("IfcMonetaryUnit"):
                     return unit