Improve variable naming

[thorbjoernl]

Change Summary

• Improved variable naming mainly in ExperimentOutput.
• Adds type hinting.
• Removes superfluous if check (EvalSetup no longer has var_mapping).
• Use namedtuple for tuple return types.

Related issue number

NA

Checklist

• [x] Start with a draft-PR
• [x] The PR title is a good summary of the changes
• [x] PR is set to AeroTools and a tentative milestone
• [x] Documentation reflects the changes where applicable
• [x] Tests for the changes exist where applicable
• [ ] Tests pass locally
• [x] Tests pass on CI
• [x] At least 1 reviewer is selected
• [x] Make PR ready to review

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.83333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.31%. Comparing base (16208c7) to head (ac80b76). Report is 2 commits behind head on main-dev.

Files	Patch %	Lines
pyaerocom/aeroval/experiment_output.py	95.65%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main-dev #1194 +/- ## ============================================ + Coverage 79.26% 79.31% +0.05% ============================================ Files 131 131 Lines 20239 20230 -9 ============================================ + Hits 16042 16046 +4 + Misses 4197 4184 -13 ``` | [Flag](https://app.codecov.io/gh/metno/pyaerocom/pull/1194/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/metno/pyaerocom/pull/1194/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno) | `79.31% <95.83%> (+0.05%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno#carryforward-flags-in-the-pull-request-comment) to find out more.

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