Rename the package to sorcha-addons

[mjuric]

This renames the package to sorcha-addons (with the python import name being sorcha_addons).

The - vs _ dichotomy comes from the convention for PyPI packages (and conda, etc.) to use dashes for word delimiters, whereas Python disallows this in module names ergo the convention to use _ instead. Note that pip install sorcha_addons, though not canonical, would still work.

After this PR is merged, I'll also change the git repo name to sorcha-addons, and someone w. the right permissions (@mschwamb ?) will need to adjust the rtd names/configuration/etc.

[mschwamb]

I'll make a new readthedocs as you can't change the slugname. But should be easy to do once the name change is in