Remove volume fraction input to kappa aerosol mode

for kappa aerosol mode, you shouldn't need to specify volume and mass fractions separately. better to specify mass fractions and densities (like for B formulation) and then calculate the volume fractions in backend instead of making the user do this.

Locate the kappa aerosol mode implementation and the existing B formulation entry point in the Julia backend. Compare how mass fractions and densities are handled there, then verify that kappa mode derives volume fractions internally and no longer requires users to provide both inputs.

Written by the indexing model from the issue text.