`matplotlib.colorbar.Colorbar` uses un-normalized axes for all mappables

Before 3.0, `matplotlib.colorbar.Colorbar` (`~.Figure.colorbar`) normalized

all axes limits between 0 and 1 and had custom tickers to handle the

labelling of the colorbar ticks. After 3.0, colorbars constructed from

mappables that were *not* contours were constructed with axes that had

limits between ``vmin`` and ``vmax`` of the mappable's norm, and the tickers

were made children of the normal axes tickers.

This version of Matplotlib extends that to mappables made by contours, and

allows the axes to run between the lowest boundary in the contour and the

highest.

Code that worked around the normalization between 0 and 1 will need to be

modified.