Move kpoints_t to electrons_t

Description

The kpoints object does not belong anymore to simulation box, but to the electronic system. The electronic Hamiltonian also knows it, which is mandatory, as some operators needs the kpoints information and can only access it through hm or mesh.

News snippet

Refactoring of the code.

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.