Implementation of the CG for the orbital minimization in RDMFT plus several improvements to the RDMFT code

Description

Several improvements of the rdmft routine. Especially, we extended the cg_solver to also treat rdmft. This feature is to be considered experimental, as it is the first implementation of rdmft in a real space code at all!

News snippet

Added in the rdmft routine the option to use conjugate gradients for the orbital optimization.

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.