Improve plot aspect handling when using cartopy

This applies to single plots and FacetGrids.

The current plotting behavior when using a projection that changes the plot aspect is as follows:

from xray.tutorial import load_dataset

ds = load_dataset('air_temperature')

ax = plt.subplot(projection=ccrs.LambertConformal())
ds.air.isel(time=0).plot(transform=ccrs.PlateCarree())
ax.coastlines()
ax.gridlines()

fg = ds.air.isel(time=slice(0, 9)).plot(col='time', col_wrap=3, transform=ccrs.PlateCarree(),
                                   subplot_kws=dict(projection=ccrs.LambertConformal()))
for ax in fg.axes.flat:
    ax.coastlines()
    ax.gridlines()

There are two problems here, I think both are related to the aspect of the subplot:

1. In the single case, the subplot aspect is correct but the colorbar is not scaled appropriately
2. In the FacetGrid case, the subplot aspects are not correct but the colorbar is.

Reproduce both examples: the single plot with a LambertConformal projection and the FacetGrid using subplot_kws. Start from the plotting and FacetGrid aspect-handling entry points, then compare subplot sizing with colorbar scaling. Done means both single and faceted plots have correct projected aspects and appropriately scaled colorbars.