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

src/ifcopenshell-python/ifcopenshell/api/structural/unassign_structural_analysis_model.py

@@ -0,0 +1,25 @@
+import ifcopenshell
+from ifcopenshell.api.owner.api import update_owner_history
+
+
+class Usecase:
+    def __init__(self, file, settings={}):
+        self.file = file
+        self.settings = {
+            "product": None,
+            "structural_analysis_model": None,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        if not self.settings["structural_analysis_model"].IsGroupedBy:
+            return
+        rel = self.settings["structural_analysis_model"].IsGroupedBy[0]
+        related_objects = set(rel.RelatedObjects) or set()
+        related_objects.remove(self.settings["product"])
+        if len(related_objects):
+            rel.RelatedObjects = list(related_objects)
+            update_owner_history(rel)
+        else:
+            self.file.remove(rel)