Add spherical_to_cartesian function

[ppegolo]

There is a cartesian_to_spherical function in featomic.clebsch_gordan, but no inverse function to transform, e.g., the predictions of spherical models back to Cartesian form.

[Luthaf]

I pushed my work in progress branch for this in the gf/spherical-to-cartesian branch on this repo if someone wants to take over!