Hide poisson_solver_is_iterative in poisson_solve

Multiple time in the code, we do:

        if (poisson_solver_is_iterative(psolver)) then
          pot_ij(1:mesh%np) = R_TOTYPE(M_ZERO)
        end if

        call X(poisson_solve)(psolver, namespace, pot_ij, rho_ij, all_nodes=.false.)

This is error prone and we should move the zeroing to the poisson_solve itself.