Removing cartopy from dependencies

[w-k-jones]

Cartopy is currently used in the plotting module, as well as being a dependency of iris. Installing cartopy with pip is difficult due to its dependency on the GEOS library. If we want to add tobac to PyPI we should look into removing cartopy from the requirements once we have replaced iris with xarray. Currently, the only use for cartopy is cartopy.crs.PlateCarree and cartopy.mpl.gridliner which would be better as external options

[freemansw1]

I agree with this, but we should keep cartopy as a dependency on the notebooks.

I've tagged this as a v1.6 item for now, but this could be good for a v1.5.x series release.

[w-k-jones]

As of the new cartopy v0.22.0 release, cartopy is easier to install using pip. Hopefully this isn't as much of an issue moving forwards