Optim grad nlcc
Optimize the calculation of the gradients of the NLCC and the atomic densities by reducing the number of spline evaluations. Also fixes a bug in the NLCC gradient calculation, in the case of some atoms not having NLCC.
Bugfix and optimization for pseudopotentials having a NLCC.
- I have checked that my code follows the Octopus coding standards
- I have added tests for all the new features added in this request.