Add multi-frequency support to HealpixMap and SphericalHarmonics

[paganol]

This PR adds multi-frequency support in maps_and_harmonics.py. In detail:

• Extends HealpixMap and SphericalHarmonics classes to handle multiple frequencies via new frequencies_ghz and nfreqs attributes.
• All methods and transforms (convolution, smoothing, interpolation, power spectra, arithmetic) now operate on multi-frequency arrays.
• Flexible convolution and smoothing: supports per-frequency and per-Stokes parameters.
• Implements frequency compatibility checks for algebraic operations.
• Updates the test suite with new multi-frequency tests.
• Improves documentation for maps_and_harmonics and input_sky modules.

In addition, input_sky now accepts an external array of frequencies, returning maps or shperical harmonics packed in a single instance of HealpixMap or SphericalHarmonics.

[paganol]

I think the PR is substantially ready; the only thing left is to decide how to handle write_fits and read_fits in SphericalHarmonics

[ziotom78]

It seems ok to me! I see you did not touch the I/O code, but this probably deserves some discussion. We might put an error if you try to read or write a multi-frequency object for the time being and address the issue in a separate PR, what do you think? In this way people can start using the feature while we figure out what is the best format (multiple files? one single huge FITS file for all the frequencies?)

Edit: Sorry, I just saw your comment:

I think the PR is substantially ready; the only thing left is to decide how to handle write_fits and read_fits in SphericalHarmonics

[ziotom78]

I just learned the existence of Sequence in typing!

[paganol]

Same here.. Claude's suggestion..

[paganol]

Hi @ziotom78, I opted for a sort of hybrid solution. The code can save FITS files in the multifrequency case, but you need to specify which frequency you want to save. read_fits is unchanged.

[ziotom78]

Hi @ziotom78, I opted for a sort of hybrid solution. The code can save FITS files in the multifrequency case, but you need to specify which frequency you want to save. read_fits is unchanged.

Oh, sure, this is the simplest and cleanest solution!