Implementing volume optimization

Description

Implementing cell volume optimization.

This allows to do a volume optimization with only a rescaling of the lattice vectors. The optimization is done on the scaling with respect to the original lattice vectors, similar to what Abinit does.

This also introduce a symmetrization of the strain and the stress tensor. For this, the nonsymmorphic symmetries are also used. Without them, hexagonal lattice do not retain the length of the basal lattice vectors identicals.

News snippet

Cell volume optimization and better treatment of the symmetrization.

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.