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
• 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?