fix: consistent dimensions for groupby outputs :wrench:

[fkroeber]

Description

This PR overwrites the default squeeze=True for xarray.groupby() by changing it to False. This ensures that the grouping dimension is always kept in the output arrays. This is important to allow indexing the grouping dimension afterwards in recipes.

Type of change

Select one or more relevant options:

• [x] Bug fix (change which fixes a bug reported in a specific issue)
• [ ] New feature (change which adds a feature requested in a specific issue)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update (improved documentation for existing functionality in the package)
• [ ] Repository update (change related to non-package files or structures in the GitHub repository)

Checklist:

• [x] I have read and understood the contributor guidelines of this project
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I ran all demo notebooks locally and there where no errors