add option to remove mid-range of base colormap in get_discrete_midpt_cmap_norm
add option to remove mid-range of base colormap in get_discrete_midpt_cmap_norm

- this allows omiting the white section in the midrange portion of
  many diverging colormaps, providing more contrast in the colors on
  either side of the midpoint.

Timothy-W-Hilton/TimPyUtils