How to aggregate parameters and hyperparameters for mixed potentials

[jrdegreeff]

Currently the parameter interface is unimplemented for LinearCombinationPotential. The simplest way to implement this would be to concatenate the results from the subpotentials and possibly annotate with some metadata to disambiguate. The problem with this is that it doesn't allow any parameters to be logically shared between subpotentials. To do this we would need a stronger notion of parameters, likely involving an extra layer of abstraction through a parameter struct. This would add a good amount of complexity, so if it's a desired feature, we should spend more time considering the design before implementing it.