To make the float zoom levels useful for Vector Tile Layer

[omanges]

Even though the zoom level trait has been made Float in the Vector Tile Layer support feature (#544), the zoom only changes on a integer value, for example, map actually changes only on 1.0, 2.0, 3.0 ..., rather than changing on any floating zoom, as floating zoom levels are possible for Vector Tile.

[davidbrochart]

Looks like we need support for https://leafletjs.com/reference-1.6.0.html#map-zoomdelta

[martinRenou]

It should be straightforward to expose this zoom_delta option to Python

[omanges]

@martinRenou assign this to me, I can try this.

[omanges]

I think we should also implement the https://leafletjs.com/reference-1.6.0.html#map-zoomsnap, zoomSnap

[omanges]

Works for me now

[zxdawn]

Is it possible to make it also work for the scroll wheel?