Combinatorics.SimpleGraph.Basic mathlib4#1883

Stream: mathlib4

[Anand Rao]

Simple graphs are now ready for porting, and I have started here. Following aesop_cat, I have created an aesop_graph tactic as a custom wrapper around aesop for proving graph-theory lemmas. I will not be working on the file for the next several hours since it is night time here, so I am notifying here in case anyone else would like to work on the file in the meantime. It is a fairly big file (around 2000 lines long) which may take a few days to complete.