Remove NASAGIBS Blue Marble example

[Conengmo]

#1882 was very helpful in showing that the currently listed example tileset doesn't work anymore. But the proposed replacement is not suitable unfortunately. Simply solve this issue by removing the example. If users want more examples, they can visit the site we mention: http://leaflet-extras.github.io/leaflet-providers/preview/

[martinfleis]

Shall we maybe point to xyzservices instead? Either to the docs (the list is here) or with the code snippet

import xyzservices.providers as xyz

xyz

Now that we depend on it, it will be more straightforward to just pass the name of the tiles rather than figuring out on leaflet-providers which URL shall be passed where and how the attribution should look like.

[Conengmo]

it will be more straightforward to just pass the name of the tiles rather than figuring out on leaflet-providers which URL shall be passed where and how the attribution should look like.

Totally agree! I think we can still keep it in the docstring of TileLayer, since it is a more advanced feature we still support, but put it lower in the order.

Shall we maybe point to xyzservices instead?

Since then xyzservices docs point to that leaflet providers preview, I'd rather link to it directly. But how about we link to both? I'll make a change.

[Conengmo]

@martinfleis I streamlined the docstring a bit more, does this look good to you? I hope it's more clear now that: users should mainly provide provider names, and that the xyzservices package is used underneath. Underneath, if they want to do more advanced stuff we still list a link to xyzservices.TileProvider, as well as how to use a custom url.

[martinfleis]

Looks good!

[Conengmo]

Thanks Martin!