feat: Add iasi mode to radiance_to_brightness_temperature

[tomas-kral]

Description

This change adds a new mode to radiance_to_brightness_temperature with support for IASI instrument.

As a contributor to the Anemoi framework, please ensure that your changes include unit tests, updates to any affected dependencies and documentation, and have been tested in a parallel setting (i.e., with multiple GPUs). As a reviewer, you are also responsible for verifying these aspects and requesting changes if they are not adequately addressed. For guidelines about those please refer to https://anemoi.readthedocs.io/en/latest/

By opening this pull request, I affirm that all authors agree to the Contributor License Agreement.

[floriankrb]

Thanks for this contribution!
I think this formula nu = 645.0 + 0.25 * (ch - 1) need some comment (a pointer to an official doc and a summary for them, maybe). When looking at the code in 10 years, people should be able to understand why we have this.

[tomas-kral]

I've added a reference to the OSCAR website which details a lot of instrument characteristics. You can find a table of channels there as well. Cheers.