Fix capacity factor timeseries in convert.py

[TimFuermann]

The method convert and aggregate was adopted, so the capacity factor timeseries is returned and all functionality, such as return capacity and per_unit are kept.

Closes # (if applicable).

Change proposed in this Pull Request

Description

Motivation and Context

How Has This Been Tested?

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [ ] I tested my contribution locally and it seems to work fine.
• [ ] I locally ran pytest inside the repository and no unexpected problems came up.
• [ ] I have adjusted the docstrings in the code appropriately.
• [ ] I have documented the effects of my code changes in the documentation doc/.
• [ ] I have added newly introduced dependencies to environment.yaml file.
• [ ] I have added a note to release notes doc/release_notes.rst.
• [ ] I have used pre-commit run --all to lint/format/check my contribution

[FabianHofmann]

thanks for the PR @TimFuermann, I will try to have a look at it soon