ENH: Adding ability to provide a img_tile found in cartopy.io.img_tiles.

[zssherman]

• Closes Sunset Stamen maps in GeoDisplay and potentially replace #727
• Tests added
• Documentation reflects changes
• PEP8 Standards or use of linter
• Xarray Dataset or DataArray variable naming follows 'ds' or 'da' naming

[zssherman]

Hmm, test plot doesn't have the track, but locally it does...

[zssherman]

Fixed.

[AdamTheisen]

@zssherman would we want to keep the Stamen keyword so we don't break anything and raise a warning that it is deprecated and will no longer be used?

[zssherman]

@AdamTheisen Ah yep good point! I can add the parameter back in and feed it into the new code

[zssherman]

@AdamTheisen I added the warning, let me know how it looks

[AdamTheisen]

@zssherman it looks like the Stamen cease to work at the end of October. We should plan to test out and maybe update/do a new release then to fully remove it.

[zssherman]

@AdamTheisen So a release right before the end of October to remove?

[AdamTheisen]

@zssherman that was my thinking. Deprecation warning now and then removal when it stops working. Open to thoughts thought

[zssherman]

@AdamTheisen That sounds good to me!