Fix some Maxwell couplings with MPI

Description

Several issues were caused by improper use of mesh_nearest_point.

This did not fix the similar GPU problem (#793 (closed))

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/hamiltonian_elec.F90

@@ -1289,6 +1289,10 @@ contains
 
     PUSH_SUB(hamiltonian_elec_update_pot)
 
+    if (this%mxll%test_equad) then
+      call set_electric_quadrupole_pot(this%mxll, mesh)
+    end if
+
     !$omp parallel private(ip, ispin)
     do ispin = 1, this%d%nspin
       if (ispin <= 2) then
@@ -1355,7 +1359,6 @@ contains
 
       ! Adding Quadrupole potential from static E-field (test)
       if (this%mxll%test_equad) then
-        call set_electric_quadrupole_pot(this%mxll, mesh)
         !$omp do simd schedule(static)
         do ip = 1, mesh%np
           this%hm_base%potential(ip, ispin) = this%hm_base%potential(ip, ispin) + this%mxll%e_quadrupole_pot(ip)