Discrete investment

[coroa]

Alternative to #371.

Changes proposed in this Pull Request

Extends lowmem lopf for generators by 3 new attributes:

• buildable
• retirable
• fom_cost

buildable allows a fixed capacity power plant to be built as proposed or not. retirable allows a fixed capacity power plant to be retired before the actual lifetime, salvaging fom_cost * years of early retirement.

This is an untested draft.

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.