uniform_gradient_d_dx Function

  function uniform_gradient_d_dx(this, dir, order) result(res)
    !* Author: Chris MacMackin
    !  Date: July 2017
    !
    ! \(\frac{\partial^{\rm order}}{\partial x_{\rm dir}^{\rm order}}{\rm field}\)
    !
    class(uniform_gradient_field), intent(in) :: this
    integer, intent(in) :: dir !! Direction in which to differentiate
    integer, optional, intent(in) :: order !! Order of the derivative, default = 1
    class(scalar_field), pointer :: res
    integer :: ord
    call this%guard_temp()
    call this%allocate_scalar_field(res)
    if (present(order)) then
      ord = order
    else
      ord = 1
    end if
    select type(res)
    class is(uniform_scalar_field)
      if (ord == 1) then
        if (dir > 0 .and. dir <= size(this%grad)) then
          res = uniform_scalar_field(this%grad(dir))
        else
          res = uniform_scalar_field(0.0_r8)
        end if
      else
        res = uniform_scalar_field(0.0_r8)
      end if
    class default
      error stop ('Non-uniform_gradient_field type allocated by '//&
                  '`allocate_scalar_field` routine.')
    end select
    call res%set_temp() ! Shouldn't need to call this, but for some
                        ! rason being set as non-temporary when
                        ! assignment subroutine returns.
    call this%clean_temp()
  end function uniform_gradient_d_dx