Dewpoint temperature entrypoints

[chpolste]

Description

Dewpoint temperature on model and pressure levels from specific humidity and pressure.

• Based on the flight levels entry point.
• Using the new earthkit.meteo.vertical API from the 1.0 release candidates.
• lnsp is archived as spectral coefficients while q is gridded. Specify grid in the requests explicitly to ensure compatibility.

Open tasks and questions

• Add tests
• Is it possible/desireable to merge the two entrypoints into a single entrypoint that can handle both ML and PL, despite the extra field required for ML?
• "dewpoint" as used in earthkit-meteo or "dew point" as used by GRIB parameter 3017 (though there are other parameters with a single-word spelling too, e.g., 168)?

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.