Bugfix: Set lsize also for the default cubic lattice when LatticeVectors block is not defined

Description

For periodic systems the simulation box can be defined either by the %LatticeParameters of %Lsize blocks. Although in the variable list is described that the default lattice is a cubic lattice, if the angles are not present the %LatticeParameters block, the lsize variable is not defined producing the sb%rlattice = 0, then sb%klattice = NaN.

Now, for the default cubic lattice is handled properly for periodic systems in simul_box_build_lattice subroutine. (I don't know why does octopus call this subroutine for finite systems)

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.