Fix sign of dipolar vector potential for phase

Description

Fixes sign of the dipolar vector potential coming from a Maxwell or external source system when coupling in dipole (phase). This is because the sign of the phase is written with the opposite sign. This makes the calculations in dipolar and full minimal coupling consistent.

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.

src/hamiltonian/mxll_elec_coupling.F90

@@ -323,7 +323,8 @@ contains
 
     case (VELOCITY_GAUGE_DIPOLE)
       call hm_base%allocate_field(mesh, FIELD_UNIFORM_VECTOR_POTENTIAL, .false.)
-      hm_base%uniform_vector_potential(1:space%dim) = hm_base%uniform_vector_potential(1:space%dim) + &
+      ! The minus sign is here is for the wrong convention of Octopus for the phase
+      hm_base%uniform_vector_potential(1:space%dim) = hm_base%uniform_vector_potential(1:space%dim) - &
         this%vec_pot_dip(1:space%dim)
 
     case (FULL_MINIMAL_COUPLING)