Reimplement nested Mixtures

[ricardoV94]

Follow up from #5438

Nested Mixtures or Mixtures using non pure RandomVariables (such as Censored variables) are not yet possible. This will require creating a dispatch to retrieve and manipulate these variables (i.e., check ndim_supp, resize), in the same way that's being done with the component RandomVariables. This should be straightforward, since these had to be implemented as methods of the SymbolicDistribution anyway...