ENH: more control on colorbar

[fmaussion]

Addresses #752 and allows to pass kwargs to colorbar. For example, it is now possible to do:

import numpy as np
import matplotlib.pyplot as plt
import xarray as xr

x, y = np.meshgrid(np.arange(12), np.arange(12))
z = xr.DataArray(np.sqrt(x**2 + y**2))
ds = z.to_dataset(name='z')

fig, ((ax1, ax2), (ax3, ax4)) = plt.subplots(2, 2, figsize=(12, 12))
ds.z.plot.contourf(ax=ax1)
ds.z.plot.contourf(ax=ax2, cbar_kwargs={'orientation':'horizontal', 'label':'MyLabel'})
ds.z.plot.contourf(ax=ax3, cbar_ax=ax4, cbar_kwargs={'orientation':'horizontal',
                                                     'label':'Funny Cbar',
                                                     'drawedges':True})
plt.tight_layout()
plt.show()

[shoyer]

This looks great -- it is possible to add a test for some of this functionality? Take a look at the existing plotting tests and see if there's anything you can do easily.

[jhamman]

one corner case that may require checking is when add_colorbar = False and cbar_ax or cbar_kwargs is specified. It might be worth checking and just raising an error. Unless you can think of a time when you want to be able to specify arguments that aren't used.

[jhamman]

This is great. I had two small comments and agree that it could use some tests.

[fmaussion]

@shoyer I added some tests and and implemented @jhamman 's comments. I am not a big fan of raising an error when keywords are set and not used, but I don't have a use case for this either so I implemented it as requested ;)

[shoyer]

This looks great. We just need a brief note for what's new now.

[fmaussion]

Right, I always forget about this one. Let me know if I should rebase, too.

[jhamman]

No need to rebase. We can squash on the github side now.

[fmaussion]

Yes, ok.

[OT]: Some people on the mpl mailing list said it was a dangerous feature of github but I didn't understand why. I guess that for large PRs it might be useful to keep the intermediate commits, but for the majority of the cases it makes the git history much clearer I think.

[shoyer]

Yeah, I'm a big fan of squash-on-merge. It's much easier than forcing contributors to master git rebase.

[shoyer]

Thanks @fmaussion !

[rvalenzuelar]

Could you give an example on how to use this functionality when using faceting plots? For example, I have a 4 column figure and I would like to have the colorbar on top.