Blockgroup modifications

[Chris7]

Enable modifications directly on blockgroups instead of paths to enable iterative engineering. I noticed on the gfa's we're exporting we need to fix how we're parsing deletions from vcfs (

[dkhofer]

Mostly looks good, I'm still trying to understand some of the details, asked some questions in slack

[dkhofer]

I think I understand enough that I'm going to approve.

One suggestion: I think prune_graph could be moved alongside build_graph in the Edge model, and prune could be a parameter to that method. Pretty much everywhere prune_graph is called, is right after build_graph in Edge or get_graph in BlockGroup