Enable some non-vanilla dark matter options from CLASS

[philbull]

CLASS is able to calculate observables for a number of more sophisticated dark matter models, like decaying CDM and annihilating DM. It would be nice to enable some of those options (in a simple way) through CCL.

[damonge]

I'd say this is up to the boltzmann solvers, and in that case the Calculator mode solves the issue. Unless others feel differently!

Will leave open for a few days and then close if no one picks it up.