plot_intensity() slice plot does not respect the square_root option.

[hadmack]

When using plot_intensity() with the slice_y_pos option, the square_root scaling is not respected. In plot_intensity.py line 152-153 the units are always W/m^2 and I**2 is plotted even if the I = √I was not computed due to the square_root=False.

Slices are only plotted correctly when square_root=True.