Configurable colorbar in iris.quickplot

[trexfeathers]

✨ Feature Request

I would like to be able to configure colorbar creation via kwargs in iris.quickplot functions.

Motivation

If I want to customise a plot using iris.quickplot, I can modify every element after creating the plot EXCEPT the colorbar (due to how Matplotlib works).

[pp-mo]

I'm not quite sure of the exact scope or intention here, but I would probably oppose any additions to the quickplot api.
IMHO iris.quickplot is a quick + simple one-stop shop. If you need more control, you should simply be using iris.plot instead.
If there is still a need for additional controls there, to do things which can't be done otherwise, then I'd say that is appropriate.

[trexfeathers]

From @SciTools/peloton:

We prefer the simpler solution of 1 new parameter: colorbar, which defaults to True. If the user wants a customised colorbar then they should use False then add their own after creating the plot.

[rcomer]

I think the work here is:

1. Add the boolean colorbar keyword to the _label function, which controls whether the colorbar is added.

2. Do the "plumbing" to expose the colorbar keyword in every public plotting function that ultimately calls _label.

3. Add tests.