Laplacian of KS orbitals incorrect at RHS boundary

[timcallow]

The laplace function in mathtools module gives some numerical errors on the RHS boundary, due to the numpy.gradient function which uses finite differences.

This contaminates the kinetic energy density and thus the total kinetic energy with a small (but significant when computing pressures) numerical error. See for example the attached, we see the k.e.d. seems to approaching an asymptotic limit but the final 3 points are incorrect.