Add missing convenience wrappers for energy, force, etc.

[jameskermode]

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import JuLIP.forces, JuLIP.gradient, JuLIP.energy
gradient(calc::ASE.ASECalculator, at::Atoms) = gradient(calc, ASE.ASEAtoms(at))
forces(calc::ASE.ASECalculator, at::Atoms) = forces(calc, ASE.ASEAtoms(at))
energy(calc::ASE.ASECalculator, at::Atoms) = energy(calc, ASE.ASEAtoms(at))

[jameskermode]

plus maybe stress too

[cortner]

Thanks you - in general I think ASE.jl needs an overhaul.

The question is whether to wait until quippy can be incorporated into this and make it all based on py3

[jameskermode]

That's fair - although much of what we're doing doesn't use quippy at all, so we could move to Py3 already which will increase pressure on me to finish that conversion.