Add a cartopy.io.img_tiles.Stamen class

[pelson]

Rationale

Replaces #1013 to generalise the Stamen class of image tiles, thus giving access to all existing and future Stamen styles (following the same URL scheme).
Also adds a more abstract GoogleWTS tiler class to allow more appropriate subclassing.

Implications

• No further Stamen subclasses to be accepted (unless there is a new Stamen URL scheme)
• All Tile classes should be updated to subclass GoogleWTS, not GoogleTiles (not done yet)
• This is my review of Add Stamen Toner and Watercolor image tiles #1013, and it closes Add Stamen Toner and Watercolor image tiles #1013. cc @manugarri

[pelson]

@QuLogic - updated as per review. Thanks for the feedback.

[QuLogic]

Looking at the warnings in the doc build, it seems like there might be a few other examples that use the StamenTerrain class.

[manugarri]

Oh nice!

[ocefpaf]

I love that this is getting the WTMS in the name but is it still worth keeping "Google" in there? Why not taking this opportunity to make the class name more generic? I believe GoogleWTS may throw users, who don't read the docs, off by thinking it only works for Google Tiles.

[pelson]

A fine observation. I agree entirely with the sentiment, but it is really hard to find a name that doesn't include the word Google - this isn't a general WMTS implementation AFAICT - it really does build upon Google mercator and uses (as far as I can tell) a non-standard y indexing scheme.

I suspect there are further abstractions that would be useful (there is a 'Microsoft' Quadtree implementation in here that really only needs a small subset of the GoogleWTS class's behaviour).

In short - I'm 100% in favour of finding a name that represents the scheme that Google implemented (I think it was them at least), but at present don't have a better suggestion.

[pelson]

@QuLogic - merge at will. GoogleWTS rename is a thing I'd happily support in a subsequent PR / at a later release.