bootstrapping updates

[tomkralidis]

• implement tests as pytest
• hookup testing to GitHub Actions CI
• add GitHub Container registry to GitHub Actions CI
• update README with install/run/release workflow
• add flake8 fixes
• cleanup JSON schemas
• update transform workflow to accept byte string (note: needs ecCodes fix)

Note that the jsonschema validation uses a local / patched jsonschema codebase that is able to work with remote references that are YAML. The schema validation is quite slow (as can be seen in the CI); so some work will be needed to optimize/caching remote schemas.