Temporal graphs

[mtorpey]

https://temporalgraph.notion.site/Definition-of-Temporal-Graphs-1da14627afc8404b81b0ef2c9ff5a8ed

This could be a nice addition for a project student.

Some thoughts:

• How do we represent a temporal graph?
• What are the questions to which we want answers?
• Can we cleverly reuse anything we already have (like homomorphisms) to model temporal graphs?

If it's out of scope for this package, perhaps it could be a new package with Digraphs as a dependency.