Support GPU backend for Skala ASE calculator

[awvwgk]

Currently the ASE calculator only supports the CPU version of Skala. If gpu4pyscf is available the ASE calculator should also expose the GPU version.

API should look somewhat like

from ase.build import molecule
from skala.ase import Skala

atoms = molecule("H2O")
atoms.calc = Skala(xc="skala", device="cuda")

f = atoms.get_forces()

cc Jose @jagarridotorres