Update to implicit namespace package

[anissa111]

PR Summary

Closes #547, replaces #557

Summary of changes:

• Converts geocat-comp to use an implicit namespace set up
  • removes geocat/__init__.py
  • changes to setup.cfg
• Makes some additions to MANIFEST.in to avoid packaging everything in the top level directory (side note, might want to revisit flat vs src-layout)

NOTE: This will have to be duplicated over on geocat-viz before being included in a release on this repo (edit: see NCAR/geocat-viz#220).

Resources:

• Setuptools package discovery
• implicit namespace package guide
• PEP420

PR Checklist

General

• Make an issue if one doesn't already exist
• Link the issue this PR resolves by adding closes #XXX to the PR description where XXX is the number of the issue.
• Add a brief summary of changes to docs/release-notes.rst in a relevant section for the next unreleased release. Possible sections include: Documentation, New Features, Bug Fixes, Internal Changes, Breaking Changes/Deprecated
• Add appropriate labels to this PR
• Make your changes in a forked repository rather than directly in this repo
• Open this PR as a draft if it is not ready for review
• Convert this PR from a draft to a full PR before requesting reviewers
• Passes precommit. To set up on your local, run pre-commit install from the top level of the repository. To manually run pre-commits, use pre-commit run --all-files and re-add any changed files before committing again and pushing.
• If needed, squash and merge PR commits into a single commit to clean up commit history

[kafitzgerald]

Looks good!

[cyschneck]

Looks good to me, glad to see it behaving with the readthedocs 👍