Add stochastic programming formulation

[Irieo]

Implement stochastic programming formulation in PyPSA

This is a draft.

This PR will close #575

With this functionality, it should be possible to replicate this stochastic problem out-of-the-box with a few lines of code: https://pypsa.readthedocs.io/en/latest/examples/stochastic-problem.html

Changes proposed in this Pull Request

a long list incoming

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.