Add an environment.yaml or conda.lock file for reproducibility

[seisman]

Currently, the exact versions of PyGMT, GMT, and other dependencies used for generating the figures are not documented.

It may be useful to include an environment.yaml file in this repository to make the notebooks more reproducible. Since we're still actively developing PyGMT and some syntax may continue to change, the environment should install the pygmt dev version from TestPyPI rather than from PyPI or conda-forge. Once the paper is accepted, we can pin PyGMT to a specific version (e.g., v0.19.0) to ensure long-term reproducibility.

[michaelgrund]

Absolutely necessary!