Splitting xc_get_vxc

Description

Splitting the calculation of the exchange-correlation potential in a part for computing the internal quantities needed and a part that actually performs the calculation using them.

Some unecesseray initializations are removed.

Moreover, the need or not to compute the Laplacian of the density is now properly treated by Octopus.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.