undo implementation attempt
.obj reference in Gvert, Gedge and Polystrips caused deepcopy problems.

Removed problem by creating reference to name
Merge branch 'dev' into feature/undo

Conflicts:
	__init__.py
	polystrips.py
clear the undo cache at start and at end of op.
Merge branch 'feature/undo' into dev

Conflicts:
	__init__.py