A minor expansion of mygfa to compute node_steps

[anshumanmohan]

mygfa.py now computes the NodeSteps_s table described in https://github.com/cucapra/notes/blob/main/Pollen/relational.md#indexes. As Adrian suggests in the note, odgi depth is now a cinch.

Just as a sanity check, here's a sketch of k.gfa.

This produces the following node depth table, which I think is correct.

[anshumanmohan]

This branch has a few more commits than the PR proposed, but I'm merging it and moving on. The additional commits are towards

• rudimentary testing (with diff instead of turnt)
• refactoring code into modules
• implementing odgi degree