First attempt at ripping out agnostic code into ifcopenshell.api namespace. See #1399.

src/ifcopenshell-python/ifcopenshell/api/owner/add_address.py

@@ -0,0 +1,11 @@
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {"assigned_object": None, "ifc_class": "IfcPostalAddress"}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        addresses = list(self.settings["assigned_object"].Addresses) if self.settings["assigned_object"].Addresses else []
+        addresses.append(self.file.create_entity(self.settings["ifc_class"], "OFFICE"))
+        self.settings["assigned_object"].Addresses = addresses