New "extend" keyword to colors.BoundaryNorm

`~.colors.BoundaryNorm` now has an ``extend`` kwarg,

analogous to ``extend`` in ~.axes._axes.Axes.contourf`. When set to

'both', 'min', or 'max', it interpolates such that the corresponding

out-of-range values are mapped to colors distinct from their in-range

neighbors. The colorbar inherits the ``extend`` argument from the

norm, so with ``extend='both'``, for example, the colorbar will have

triangular extensions for out-of-range values with colors that differ

from adjacent colors.

.. plot::

import matplotlib.pyplot as plt

from matplotlib.colors import BoundaryNorm

import numpy as np

# Make the data

dx, dy = 0.05, 0.05

y, x = np.mgrid[slice(1, 5 + dy, dy),

slice(1, 5 + dx, dx)]

z = np.sin(x) ** 10 + np.cos(10 + y * x) * np.cos(x)

z = z[:-1, :-1]

# Z roughly varies between -1 and +1.

# Color boundary levels range from -0.8 to 0.8, so there are out-of-bounds

# areas.

levels = [-0.8, -0.5, -0.2, 0.2, 0.5, 0.8]

cmap = plt.get_cmap('PiYG')

fig, axs = plt.subplots(nrows=2, constrained_layout=True, sharex=True)

# Before this change:

norm = BoundaryNorm(levels, ncolors=cmap.N)

im = axs[0].pcolormesh(x, y, z, cmap=cmap, norm=norm)

fig.colorbar(im, ax=axs[0], extend='both')

axs[0].axis([x.min(), x.max(), y.min(), y.max()])

axs[0].set_title("Colorbar with extend='both'")

# With the new keyword:

norm = BoundaryNorm(levels, ncolors=cmap.N, extend='both')

im = axs[1].pcolormesh(x, y, z, cmap=cmap, norm=norm)

fig.colorbar(im, ax=axs[1]) # note that the colorbar is updated accordingly

axs[1].axis([x.min(), x.max(), y.min(), y.max()])

axs[1].set_title("BoundaryNorm with extend='both'")

plt.show()