Use low-level AtomsCalculator interface internally

[mfherbst]

Instead of using the high-level energy_forces_virial we should use the low-level calculate((Energy(), Forces(), Virial(), ...) which allows more optimisations for expensive calculators.

[tjjarvinen]

My opinion here is that this should be done on AtomsCalculators level. So that energy_forces_virial calls low-level call, if not overwritten by the calculator implementer. This is not what it does currently.

You can, however, write for your calculator that energy_forces_virial calls the low-level interface and thus get what you are after.

[mfherbst]

This is not what it does currently.

I agree, that shoud be done over there instead. I have indeed followed your suggestion and provided a custom method for my calculator.