Implement add / remove load groups, and half implement structural activities. Thanks Jesusbill and ihabelaghoury

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

@@ -0,0 +1,16 @@
+import ifcopenshell.api
+
+
+class Usecase:
+    def __init__(self, file, **settings):
+        self.file = file
+        self.settings = {"load_group": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        # TODO: do a deep purge
+        for inverse in self.file.get_inverse(self.settings["load_group"]):
+            if inverse.is_a("IfcRelAssignsToGroup") and len(inverse.RelatedObjects) == 1:
+                self.file.remove(inverse)
+        self.file.remove(self.settings["load_group"])