Adding custom variables for cloud study

[LisaBock]

Description

For a cloud study I would need some new custom variables which are

• cloud top pressure (ctp)
• cloud effective radius (reff)
• temperature at 1000 hPa (ta1000)
• vertical velocity at 700 hPa (wap700)

Closes #2481

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Checklist

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• [x] 🧪 The new functionality is relevant and scientifically sound
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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.70%. Comparing base (0a58be1) to head (0f6c30f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2482 +/- ## ======================================= Coverage 94.70% 94.70% ======================================= Files 249 249 Lines 14114 14114 ======================================= Hits 13367 13367 Misses 747 747 ```

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