Ready for 0.2.1 PyPI/GitHub release

[yucongalicechen]

PyPI/GitHub rc-release preparation checklist:

• All PRs/issues attached to the release are merged.
• All the badges on the README are passing.
• License information is verified as correct. If you are unsure, please comment below.
• Locally rendered documentation contains all appropriate pages, including API references (check no modules are
  missing), tutorials, and other human-written text is up-to-date with any changes in the code.
• Installation instructions in the README, documentation, and the website (e.g., diffpy.org) are updated.
• Successfully run any tutorial examples or do functional testing with the latest Python version.
• Grammar and writing quality are checked (no typos).
• Install pip install build twine, run python -m build and twine check dist/* to ensure that the package can be built and is correctly formatted for PyPI release.

Please mention @sbillinge here when you are ready for PyPI/GitHub release. Include any additional comments necessary, such as version information and details about the pre-release here:

PyPI/GitHub full-release preparation checklist:

• Create a new conda environment and install the rc from PyPI (pip install <package-name>==??)
• License information on PyPI is correct.
• Docs are deployed successfully to https://www.diffpy.org/<package-name>.
• Successfully run all tests, tutorial examples or do functional testing.

Please let @sbillinge know that all checks are done and the package is ready for full release.

conda-forge release preparation checklist:

• Ensure that the full release has appeared on PyPI successfully.
• New package dependencies listed in conda.txt and test.txt are added to meta.yaml in the feedstock.
• Close any open issues on the feedstock. Reach out to @bobleesj if you have questions.
• Tag @sbillinge and @bobleesj for conda-forge release.

Post-release checklist

• Run tutorial examples and conduct functional testing using the installation guide in the README. Attach screenshots/results as comments.
• Documentation (README, tutorials, API references, and websites) is deployed without broken links or missing figures.