Add new column fom_cost to all components with capital costs

[lisazeyen]

Closes #371 .

Changes proposed in this Pull Request

This PR should add an additional column to all components with capital costs. The capital costs are still including FOM + annualised investment costs. The information about the FOM fom_cost can be only used to calculated savings compared to the full capital costs in case of early retirement.

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.

[Alex-Neve]

Please consider writing "Fixed Operation and Maintenance costs" as description of fom_cost, at least to facilitate searches in the file and in the documentation.

The early retirement could also stem from a constraint on the max GHG emissions, in which case the savings not only stem from FOM costs decrease but also from VOM costs and fuel costs decrease.

[Alex-Neve]

See my comment for generators.csv: please consider using the description "Fixed Operation and Maintenance costs" to be more explicit and facilitate searches for this parameter.

[Alex-Neve]

For each component CSV file, please consider using the description "Fixed Operation and Maintenance costs. Applies to an asset that is active, whether it is ON or OFF. Required to calculate total cost of electricity and enable decommissioning." to be more explicit and facilitate searches for this parameter.

The fixed O&M cost calculation should also be implemented downstream (spatial.py, linopf.py, opf.py, abstract.py, global_constraints.py, optimize.py, pf.py and statistics.py). Do you want to implement it?
If not I could try to create a separate PR to do it, following yours.

The doc/release_notes.rst should also be updated to indicate the changes.