Make l1b_data class compatible w/ future xarray by defining __slots__

[j-haacker]

Is your feature request related to a problem? Please describe.
xarray warns:

FutureWarning: xarray subclass l1b_data should explicitly define slots

Describe the solution you'd like
Adapt class l1b_data such that it will not break in future xarray releases.

Note: this is not related to issues with xarray v2024.3.0

[j-haacker]

It seems defining slots can save memory if used correctly.

A simple solution might be to just add __slots__ = (,) to the class, but maybe one should take the effort to restructure init to only load used fields -- and to define slots for those.