DRAFT: Support `ccrs.Geodetic` source projection, add Polygon plot to Grid

[philipc2]

Closes #1230

Overview

• Adds support for using Cartopy's Geodetic projection as a source projection, allowing for better visualization of grids on the surface of a sphere and for handling splitting along the dateline: https://github.com/holoviz/geoviews/issues/107
• Using the Geodetic projection is especially useful for coarser grids where the distortion is more noticeable, and over the poles
• Implements a Grid.plot.polygons() method that plots shaded polygons using the index of each face.

Examples

No Source Projection

Geodetic Source Projection