Update map_iwave case to use new diagnostics
This case has uncovered that the BPE calculation returns incorrect values when a Chebyshev grid is used on a field scale. The BPE calculation uses the quadrature weights as the volumes of each cell. This is true when the grid is regular, but incorrect for Chebyshev grids. To first approximation the Cheb quadrature weights are close to the volumes of the cells. This leads to the BPE calculation being accurate to 8 significant figures. 8 sig figs is not enough on fields scales since the magnitude of the PE can be very, very large. The difference of the BPE from the PE can thus be smaller than these 8 sig figs.