truncate_colormap parameter n isn't working as intended

[kafitzgerald]

While I was updating truncate_colormap in viz, I noticed the parameter n wasn't working as intended. It would need to be specified as a parameter in mpl.colors.LinearSegmentedColormap.from_list rather than in np.linspace in order to specify the number of color values in the new colormap.

I thought maybe this was related to a change in matplotlib, but that doesn't appear to be the case.

Interestingly there are workaround(s) for this in the examples gallery. For example, in NCL_vector_4.py the colormap is truncated with n=16, but this doesn't work so then get_cmap is used to re-register it and give it 16 levels as desired or when using contourf the number of levels is assigned so it doesn't matter that truncate_colormap doesn't work as intended.

We should probably also clean up the colormap registration and deprecated colormap functions in examples. I'll log an issue over there.