added afforestation via manual input

[BertoGBG]

Closes # (if applicable).

Changes proposed in this Pull Request

added afforestation costs in /inputs/manual_input.csv
output files have been updated as well

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• [Y] Data source for new technologies is clearly stated.
• Newly introduced dependencies are added to environment.yaml (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• [Y] I consent to the release of this PR's code under the GPLv3 license.

[fneum]

Could you add a little bit more details on how the investment value of 7749 €/ha was obtained (in comment column)? E.g. which information from which table, which USD to EUR conversion factor.