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

src/ifcopenshell-python/ifcopenshell/api/pset/add_qto.py

@@ -0,0 +1,25 @@
+import ifcopenshell
+import blenderbim.bim.schema  # TODO: refactor
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {"product": None, "Name": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        if self.settings["product"].is_a("IfcObject"):
+            qto = self.file.create_entity(
+                "IfcElementQuantity", **{"GlobalId": ifcopenshell.guid.new(), "Name": self.settings["Name"]}
+            )
+            self.file.create_entity(
+                "IfcRelDefinesByProperties",
+                **{
+                    "GlobalId": ifcopenshell.guid.new(),
+                    # TODO: owner history
+                    "RelatedObjects": [self.settings["product"]],
+                    "RelatingPropertyDefinition": qto,
+                }
+            )