Draft: Merge dcdm

Implement decaying dark matter and dark radiation.

You probably don't want this extra field in the main code. Maybe worth adding a compile flag? It's similar to the neutrino weighting issue.

Do we need this on a particle by particle basis? Isn't that going to be a global multiplier of gp->mass ?

@dc-elbe1 we should probably merge this before it's too late and fully lost.