Improved symmetrizer

Description

Improving the symmetrization code to be more efficient and adding a routine for symmetrizing wavefunctions. This routine will be used later. By storing some mapping function, computed when the symmetrizer object is initialized, the symmetrization is more efficient. This is highly relevant when the the symmetrization is called many times, e.g. on many wavefunctions. This is part of the merging of hybrid functionals.

News snippet

More efficient symmetrization of real-space quantities.

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.