Implement ability to add SI units

src/ifcopenshell-python/ifcopenshell/api/unit/add_si_unit.py

@@ -0,0 +1,9 @@
+class Usecase:
+    def __init__(self, file, **settings):
+        self.file = file
+        self.settings = {"unit_type": "LENGTHUNIT", "name": "METRE"}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        return self.file.create_entity("IfcSIUnit", UnitType=self.settings["unit_type"], Name=self.settings["name"])