CF timeseries per gridcell

Description

It can be useful to get access to the hourly capacity factor per grid cell. this small addition to convert_and_agregate allows it.

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

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