Fix Maxwell problems/add feature: output of dipolar vec_pot, input envelope function from expression, MPI bug in dipole field calc

Description

Implement spatial envelope function from expression (was parsed but never used). Fix maxwell_dipole_field output from the electronic system that was broken for the dipolar vector potential. Fix calculation of maxwell dipole field using center of mass, using wrong array in mesh%allreduce, that went unnoticed in previous MR.

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.