Changes proposed in this Pull Request

Adds support to set options via environment variables. E.g. system wide PYPSA_PARAMS__OPTIMIZE__SOLVER_NAME=gurobi in your bashrc. Environment variables bring less overhead than own config files (like matplotlib etc.)

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• Unit tests for new features were added (if applicable).
• A note for the release notes docs/release-notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.