New and improved 3D bar plots

We fixed a long standing issue with incorrect z-sorting in 3d bar graphs.

It is now possible to produce 3D bar charts that render correctly for all

viewing angles by using `.Axes3D.bar3d_grid`. In addition, bar charts with

hexagonal cross section can now be created with `.Axes3Dx.hexbar3d`. This

supports visualisation of density maps on hexagonal tessellations of the data

space. Two new artist collections are introduced to support this functionality:

`.Bar3DCollection` and `.HexBar3DCollection`.

.. plot::

:include-source: true

:alt: Example of creating hexagonal 3D bars

import matplotlib.pyplot as plt

import numpy as np

fig, (ax1, ax2) = plt.subplots(1, 2, subplot_kw={'projection': '3d'})

bars3d = ax1.bar3d_grid([0, 1], [0, 1], [1, 2], '0.8', facecolors=('m', 'y'))

hexbars3d = ax2.hexbar3d([0, 1], [0, 1], [1, 2], '0.8', facecolors=('m', 'y'))

plt.show()