allow passing TileLayer to Map

[Conengmo]

Looking at #1316 and #1033, this seemed like an obvious improvement that doesn't actually increase the parameter footprint of the Map class.

With this change a user doesn't have to do the following:

m = Map(tiles=None)
TileLayer().add_to(m)

but can do this directly:

m = Map(tiles=TileLayer())

Discussion

I still think we should be careful to add more parameters to Map, to prevent it from becoming a god-class. If Map has the same parameters as TileLayer, we are doing something wrong. I think we should stick with Map having the tiles argument as a convenience, and have users use TileLayer when they want to do more advanced things.

Maybe a way to make this more clear is by allowing the tiles argument of Map to only be one of the pre-defined string arguments or a TileLayer, but not a custom url. Users wanting to use a custom url are thus forced to use TileLayer. That way we could also scrap the attr argument of Map.

Since that would mean breaking changes for some users, I don't think we should actually go ahead with that.

[ocefpaf]

Looks like a great improvement IMO and it is not really an extra parameter we are adding, just a valid input for an existing one.