Allow disabling undo on save

The transaction system does not scale on large scale or complex remove_deep deletions. So if you want to delete a mesh with many thousands of vertices, it becomes very, very slow. Unfortunately we cannot also clear Blender's undo history, so this advanced feature should be used carefully.

Tested:

 - deleting 250 average objects: 264s before, 246s after (6.8% faster)
 - deleting 1 object with 6,000 vertices: 5 minutes and counting (basically impractical) before, 11s after

src/ifcopenshell-python/ifcopenshell/file.py

@@ -203,7 +203,8 @@ class file(object):
             self.history.pop(0)
 
     def begin_transaction(self):
-        self.transaction = Transaction(self)
+        if self.history_size:
+            self.transaction = Transaction(self)
 
     def end_transaction(self):
         if self.transaction: