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

src/ifcopenshell-python/ifcopenshell/api/document/add_reference.py

@@ -0,0 +1,14 @@
+import ifcopenshell
+
+class Usecase:
+    def __init__(self, file, settings={}):
+        self.file = file
+        self.settings = {}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        id_attribute = "ItemReference" if self.file.schema == "IFC2X3" else "Identification"
+        return self.file.create_entity("IfcDocumentReference", **{
+            id_attribute: ifcopenshell.guid.new()
+        })