Add ducc0 interpolation option to scan_map

[paganol]

We should extend scan_map with an option to use the spherical-harmonic–based interpolation provided by ducc0.sht.experimental.synthesis_general

The implementation is straightforward.
The only part of scan_map that requires modification is the handling of the input maps before interpolation:
1. Accept either:
- a standard HEALPix map, or
- already-computed alm coefficients.
2. (optionally) If a pixel map is provided, convert it to alm. If we prefer not having this option if a pixel map is provided and the ducc0 interpolation is asked, rise an error.
3. Call ducc0.sht.experimental.synthesis_general to evaluate the field at the pointing directions.
4. Keep the rest of the scan_map logic unchanged.

[mreineck]

Just as a mental note: do we potentially want to perform the corresponding adjoint operation wherever TOD are binned into a map?