feat: use .walk file in complexity command

[aryarm]

very draft, rough as sandpaper, thoroughly untested, needs much refactoring

todo:

• write more tests to check whether loading from .walk file works the same as loading walks from gfa
• try to get a gfa for a region that is less than 500KB, so that we can use it for testing:
  RCHD/RCHE locus: hg38 query: chr1:25240000-25460000
  • this is unfortunately too big (a few MBs)
  • I also tried chr1:25368857-25368858 which seems to capture the entire loop but also results in a GFA that's 2.8 MB
  • removing the walks from that GFA brings it down to 476 KB! which is great, but also it would be nice if I could keep the walks in the file...
• load node indices as integers instead of strings. will require changing some of the test data
• write tests to check whether it matters that some walks might pass through a node more than once
• benchmark and consider whether a different data structure for walks might be better
• move classes in graph_utils and gbz_utils to data module?