simplify tree "rebalancing"

[doudou]

We now unconditionally ensure that the graph is a tree (as it should),
by always removing the parent from the target node, and reconnecting
said node to the new transformation.

The very complex implementation currently there did not seem to be needed.

[arneboe]

My new test-case works but now some of the old ones fail.

testTransformerGraph.cpp(149): fatal error: in "vizkit3d_TransformerGraph/it_appends_new_edges_to_the_existing_graph": critical check it != description.end() has failed
testTransformerGraph.cpp(149): fatal error: in "vizkit3d_TransformerGraph/it_automatically_changes_the_node_shape_when_needed": critical check it != description.end() has failed
testTransformerGraph.cpp(138): fatal error: in "vizkit3d_TransformerGraph/it_handles_loops_gracefully_when_reshaping": critical check description.size() == expected_graph_size has failed [1 != 4]

[planthaber]

This has not been merged?

[doudou]

No, since there are some tests that are not passing.