JOSS revisions

Changes and additions to code based on JOSS review process: openjournals/joss-reviews#1969.

See the CHANGELOG or closed review Issues for specifics.

Data and models in assets.zip are unchanged. Users should still download from v0.2 release: https://github.com/rgmyr/corebreakout/releases/tag/v0.2

First release with `assets`

Primary purpose of this release is to attach assets.zip archive containing pretrained models and labeled BGS training + testing data.

See the CHANGELOG for other details.