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

src/ifcopenshell-python/ifcopenshell/api/spatial/assign_container.py

@@ -0,0 +1,42 @@
+import ifcopenshell
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "product": None,
+            "relating_structure": None,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        contained_in_structure = self.settings["product"].ContainedInStructure
+        contains_elements = self.settings["relating_structure"].ContainsElements
+
+        if contains_elements and contained_in_structure and contained_in_structure[0] == contains_elements[0]:
+            return
+
+        if contained_in_structure:
+            related_elements = list(contained_in_structure[0].RelatedElements)
+            related_elements.remove(self.settings["product"])
+            if related_elements:
+                contained_in_structure[0].RelatedElements = related_elements
+            else:
+                self.file.remove(contained_in_structure[0])
+
+        if contains_elements:
+            related_elements = list(contains_elements[0].RelatedElements)
+            related_elements.append(self.settings["product"])
+            contains_elements[0].RelatedElements = related_elements
+        else:
+            contains_elements = self.file.create_entity(
+                "IfcRelContainedInSpatialStructure",
+                **{
+                    "GlobalId": ifcopenshell.guid.new(),
+                    # TODO "OwnerHistory": None
+                    "RelatedElements": [self.settings["product"]],
+                    "RelatingStructure": self.settings["relating_structure"],
+                }
+            )