Add Thomas Purcell to contributors.md

[tpurcell90]

Suggested by @janosh to remove first time contributor status in #562

Summary

Add myself to the contributors list

Additional dependencies introduced (if any)

• None

TODO (if any)

None

Checklist

Work-in-progress pull requests are encouraged, but please put [WIP] in the pull request
title.

Before a pull request can be merged, the following items must be checked:

• Code is in the standard Python style.
  The easiest way to handle this is to run the following in the correct sequence on
  your local machine. Start with running black on your new code. This will
  automatically reformat your code to PEP8 conventions and removes most issues. Then run
  ruff.
• Docstrings have been added in the Numpy docstring format.
  Run ruff on your code.
• Type annotations are highly encouraged. Run mypy to
  type check your code.
• Tests have been added for any new functionality or bug fixes.
• All linting and tests pass.

Note that the CI system will run all the above checks. But it will be much more
efficient if you already fix most errors prior to submitting the PR. It is highly
recommended that you use the pre-commit hook provided in the repository. Simply run
pre-commit install and a check will be run prior to allowing commits.

[JaGeo]

@janosh Very good suggestion, as always! Thanks!