support calculator lists

[alinelena]

will be nice to be able to support for a structure, structures calculator lists.

eg, I want to have for a system, energies, forces stresses with mace and sevennet
we will need to think of best approach

[ElliottKasoar]

We may be able to extend/subclass one of the mixing calculator classes, or at least implement something similar to the Mixer base class, without the need for weights.