Permit descending colorbar levels

[lukelbd]

This PR will permit descending colorbar levels. This is not restoring a native matplotlib feature; it is a new proplot feature. I'm adding this simply because I wanted it for one of my plots, and there's no good reason proplot/matplotlib should have this limitation.

The following example demonstrates how this works for both evenly spaced and unevenly spaced levels:

import proplot as plot
import numpy as np
data = np.random.rand(20, 20).cumsum(axis=0)
for levels in (
    np.arange(0, 15, 1)[::-1],  # with Normalizer
    [15, 10, 5, 2, 1, 0],  # with LinearSegmentedNorm
):
    f, ax = plot.subplots()
    ax.pcolormesh(data, levels=levels, colorbar='b')

[lukelbd]

Here's an example showing this works with parametric plots too:

import proplot as plot
values = np.linspace(0, 9, 10)[::-1]  # with Normalizer
f, ax = plot.subplots()
x = y = np.arange(10)
ax.parametric(x, y, lw=5, cmap='Fire', values=values, colorbar='b')

[lukelbd]

Please note that due to limitations of matplotlib's contouring algorithm (matplotlib/matplotlib#5477), this will not work for contour plots. I guess this used to be a bug when that discussion was going on, but now matplotlib explicitly enforces increasing contour levels.