Edge repr layers

[narnolddd]

What changes were proposed in this pull request?

Add layers to the edge repr to show what layers an edge/exploded edge is present in, e.g.

Edge(source=A, target=B, earliest_time=1, latest_time=6, properties={greeting: yo, greeting: yo}, layer(s)=[layer 1, layer 2])

Why are the changes needed?

Useful to be displayed in the edge summary.

Does this PR introduce any user-facing change? If yes is this documented?

No.

How was this patch tested?

Added a python test file to check the output seems sane on the two types of graph.

Are there any further changes required?

Issue #1808 is a bit related since it involves how properties are displayed across layers. Could do a two-in-one if the solution is not too hard.