xref #19476

There is likely lots to iron out, and many tests and documentation that would need adding, but I thought I would open this as a proof of concept for images (or more generally mappables) with units. Things that come to mind are:

• Unit converters expect an axis argument, I'm not sure what to do about this when we have a mappable and not an axis
• munits.registry.get_converter expects a 1D sequence as input, not 2D, there's probably a way to modify this though

This allows things like this image, where the correct formatter is automatically set on the colorbar:

from datetime import datetime, timedelta
import matplotlib.colors as mcolor
import matplotlib.pyplot as plt

data = [[datetime.now(), datetime.now() - timedelta(seconds=1)],
        [datetime.now(), datetime.now()]]

fig, ax = plt.subplots()
im = ax.imshow(data)
fig.colorbar(im)

plt.show()