Rewrite report aggregation of indicator to use mean value of indicator result class values

[matthiasschaub]

Description

• rewrite report aggregation of indicator to use mean value of indicator result class values
• make indicator/layer combinations an attribute
• fix typing in report base class

Corresponding issue

Closes #322, #263

Checklist

• I have updated my branch to main (e.g. through git rebase main)
• My code follows the style guide and was checked with pre-commit before committing
• I have commented my code
• I have added sufficient unit and integration tests
• I have updated the CHANGELOG.md
• waiting for Generalize result values of all Indicators by introducing a result class value #369

[matthiasschaub]

I had a look at the code, run all the tests and tested it manually a bit. The changes look good to me!
I left one minor comment.

I think what would be good to have are tests for the blocking features. Maybe extending test_base_report with those tests would be a good idea.