slow-odgi: Make a little top-level CLI

[sampsyo]

This is purely a quality-of-life thing, but it would be cool to make a top-level CLI entry point for the slow-odgi commands. In particular, what is currently python3 slow_odgi/flip.py could become slow-odgi flip, i.e., we'd make an installable executable called slow-odgi with several subcommands.

The concrete steps for this would be:

• Add an __init__.py to the slow_odgi directory to make it into a Python package.
• Adjust the imports in slow_odgi/*.py to use package-relative imports. For example, use from . import mygfa instead of import mygfa.
• Somewhere (in __init__.py or a new __main__.py), make a new main function that dispatches to all of the various algorithms. argparse supports subcommands; we could also try Click instead.
• Add metadata to make the slow_odgi package installable (with an executable). As you probably know, my favorite tool for this is Flit, but there are other options available.

Probably best to ignore this until after #29 and #31 are merged.

[anshumanmohan]

Going to wait until #48 lands, since the changes proposed here will create a large diff and create further opportunities for tidying! Those may clash with the cleanup of 48.