implement power spectra

[iluise]

Description

code from anemoi.models (https://github.com/ecmwf/anemoi-core/blob/main/models/src/anemoi/models/layers/)

• implement SphericalHarmonicTransform
• implement InverseSphericalHarmonicTransform (mostly for sanity check)
• implement UKMet office fft transform (taken from script in evaluate/example_extras/power_spectra)

Use it as:

evaluation:
  metrics  : 
    - rmse
    - psd:
        psd_method: "set"                    # "sht" (SHT-based) or "fft" (regrid FFT)
        psd_regrid_resolution: 1.0           # degrees, only for fft method
        grid_type: "octahedral"              # "octahedral" (default), "regular", "reduced".

NB. for N320 grids we need to use reduced instead of octahedral. I tested it for both O96 and N320 grids.

The plots are in <summary_dir>/psd. The code produces one plot per forecast step.

I know it would be more appropriate to have them under results/run_id/plots/psd, but I followed the same workflow of the qq_analysis metric and I will change this in another PR that restructures this part.

Sanity check:

I implemented the same sanity check as what done in anemoi.models, so check that applying the SphericalTransform and its InverseSphericalTransform starting from random noise gives the identity.

Issue Number

Closes #2176

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Checklist before asking for review

• I have performed a self-review of my code
• My changes comply with basic sanity checks:
  • I have fixed formatting issues with ./scripts/actions.sh lint
  • I have run unit tests with ./scripts/actions.sh unit-test
  • I have documented my code and I have updated the docstrings.
  • I have added unit tests, if relevant
• I have tried my changes with data and code:
  • I have run the integration tests with ./scripts/actions.sh integration-test
  • (bigger changes) I have run a full training and I have written in the comment the run_id(s): launch-slurm.py --time 60
  • (bigger changes and experiments) I have shared a hegdedoc in the github issue with all the configurations and runs for this experiments
• I have informed and aligned with people impacted by my change:
  • for config changes: the MatterMost channels and/or a design doc
  • for changes of dependencies: the MatterMost software development channel