feat: added simple check to `graph_from_literal()`

[schochastics]

Fix #824

The PR implements the suggested additional check if a graph is simple or not. This required a lot of adjustments in the tests given that this means that the edge order was changed. I could imagine that this might lead to issues downstream so keeping this for after the 2.2 release.

cc @szhorvat is this what you intended?

[krlmlr]

Too risky, let's wait.