`clebsch_gordan`: extend what can be passed to `selected_keys`

[jwa7]

When using the clebsch_gordan.DensityCorrelations calculator, one can select keys to be output at each CG iteration step by specifying values of o3_lambda and o3_sigma in the selected_keys parameter. This avoids wasting computation on CG combinations to produce blocks that aren't required for a given symmetry of a desired learning target.

For a basis set expansion of scalar fields, the target basis set definition (i.e. lmax and nmax) can be very different depending on the chemical species. It would be useful to extend the functionality of the DensityCorrelations (and future) calculators to allow switching off certain combinations of (o3_lambda, center_type), and thus switch off computation of blocks that aren't required for a certain target basis.