normalise_fluence Subroutine

        subroutine normalise_fluence(grid, array, nphotons)
        !! normalise fluence in the Lucy 1999 way
            
            use gridMod
            use constants, only : sp

            !> grid class
            type(cart_grid), intent(in) :: grid
            !> array to normalise
            real(kind=sp),   intent(inout) :: array(:, :, :)
            !> number of photons run
            integer,         intent(in) :: nphotons
            
            real(kind=wp) :: xmax, ymax, zmax
            integer       :: nxg, nyg, nzg

            nxg = grid%nxg
            nyg = grid%nyg
            nzg = grid%nzg
            xmax = grid%xmax
            ymax = grid%ymax
            zmax = grid%zmax

array  = array * ((2._sp*xmax*2._sp*ymax)/(nphotons * (2._sp * xmax / nxg) * (2._sp * ymax / nyg) * (2._sp * zmax / nzg)))

        end subroutine normalise_fluence