WIP: Support for tile services in local coordinate systems

[Huite]

This currently adds:

• Some lightly edited functions, which take some additional data from the Provider.
• projecttile rather than mercantile for some tile handling, located here: https://github.com/Huite/projecttile
  It doesn't have dependencies currently, except if you'd want to generate a list of providers yourself in which case pyproj and owslib are necessary.
• Adds a script for fetching the data of a WMTS services, which uses scripts/parse_leaflet_providers.py
• A notebook with two basic examples

I initially hoped that I wouldn't have to explicitly set all providers. My hope was that you could just present a WMTS url, take a peek at what it's serving, and pick a layer. There's some issues with this:

• it requires owslib, although it's not a very heavy dependency
• it requires pyproj for one the fly reprojection of the bounding box, although since rasterio is already required it's probably also not a big deal(?)
• it doesn't seem straightforward to extract a URL using owslib, so I couldn't get the retryer to work; you'd have to use owslib, but it does its own things with requests.
• However, by far the worst: it appears that a good deal of WMTS services play pretty loose with the standards (for as far as a standard exists), so I couldn't get owslib to fetch the tile.

Instead, I've now chosen to keep it a little simpler and more explicit by generating a list of providers. A user can just choose a provider, or create one, and it should work. I think the upshot is also that there's very fine control: it the WMTS URL doesn't comply to standards, you can just edit the URL template to whatever works.

WIP

I haven't added any tests yet, because I'd like to get a little bit feedback. To make it maximally clear what changes are necessary, I've just copy-pasted and edited some functions and appended _wmts. This is not my preferred interface, it's just for communications purposes.

Maybe it's most straightforward to create another TileProvider, WebMapTileServiceProvider, and dispatch on the type?

In overview, the changes required:

• _calculate_zoom requires boundaries: currently it's always 360:

    zoom_lon = np.ceil(np.log2(360 * 2.0 / lon_length))
    zoom_lat = np.ceil(np.log2(360 * 2.0 / lat_length))
    zoom = np.max([zoom_lon, zoom_lat]

• I need a _clamp zoom_level to avoid getting invalid URLs (doesn't that come up currently, if you plot a very small feature?)
• mercantile -> projecttile for .tiles
• Skip conversions of WebMercator <-> Lat-Lon
• A s_crs for warp_tiles?

If you do make a decision on if and how to include these changes, I'll be happy to setup CI properly, also for projecttile.