TransformGraph: Shouldn't a trivial, empty path between a space and itself be valid?

[Tomaz-Vieira]

I find it surprising that it is an error to search for a path between a graph vertex and itself:

graph = TransformGraph[np.ndarray]()

graph.add_transform(Scale(scale=[1,2], spaces=Spaces("A", "B")))

try:
    a_to_a = graph.get_sequence("A", "A")
except ValueError:
    ...

If feels like get_sequence(A, A) should succeed with a 0-length sequence (or even an Identity), and that would remove many error-handling paths for the consuming code.

[clbarnes]

Yes, this constraint could be removed. I don't hate making users check that they are not doing no-op transforms but I suppose it's not our problem if they want to.

Issue at #77