deprecated matplotlib colormaps functions in NCL_vector_4.py

[kafitzgerald]

plt.cm.register_cmap and plt.cm.get_cmap, both in NCL_vector_4.py, are both deprecated as of version 3.7 of matplotlib. We should probably update these and clean up that section of the script. The re-registration of colormaps is likely also problematic in more recent versions of matplotlib.

[kafitzgerald]

If we wait until NCAR/geocat-viz#124 is addressed in viz we can clean this up even more and truncate_colormap will work properly.