Colorbars for variables on pressure levels

[jwarner8]

Currently, there are no entries for pressure level variables. If we set a variable vmin, vmax, for say geopotential height, we would need a huge range to cater for this variable on 200hPa vs 850hPa, and so the users wouldn't be able to identify any features. Currently the colorscale automatically scales to each plot, so can change as the user scrolls through time. I thought about potential solutions such as expanding the colorbar read for cmap, levels, norm = _colorbar_map_levels(cube.name(),level=int(cube.coord('pressure').points[0])), but we would end up with a huge list in colorbar for every pressure level that a user might choose.

[Sylviabohnenstengel]

By coincidence have implemented something along those lines in #1140. but this would blow up the json file :-(

Maybe we need to think around interpolation for the colourbar.