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

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

@@ -0,0 +1,27 @@
+import blenderbim.bim.schema  # TODO: refactor
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {"product": None, "pset": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        to_purge = []
+        should_remove_pset = True
+        for inverse in self.file.get_inverse(self.settings["pset"]):
+            if inverse.is_a("IfcRelDefinesByProperties"):
+                if not inverse.RelatedObjects or len(inverse.RelatedObjects) == 1:
+                    to_purge.append(inverse)
+                else:
+                    related_objects = list(inverse.RelatedObjects)
+                    related_objects.remove(self.settings["product"])
+                    inverse.RelatedObjects = related_objects
+                    should_remove_pset = False
+        if should_remove_pset:
+            self.file.remove(self.settings["pset"])
+        for element in to_purge:
+            self.file.remove(element)
+        # TODO: implement deep purging