Use networkx to delete downstream components

[trevorhardy]

Related to #102. .del_objects() should not just delete objects that are children of the deleted objects but also those that are electrically connected using link objects (e.g. transformers, lines).

This will only work for radial networks (acyclic graphs); for meshed networks (cyclic graphs) we should only delete the existing component. Doing this right will likely take some fancy networkx work to make sure we're doing the safe and sensible thing. Lots of edge cases to think through.

[trevorhardy]

Feature implemented not using networkx; waiting to be merged into develop branch.

[trevorhardy]

Current version now works with parent-child relationships but not those connected via lines.