Reduce zoom delta in LayeredMap

[HansKallekleiv]

We are currently using integer only zoom in LayeredMap.
This gives quite large jump in zoom levels when zooming in the view.
It would be useful if the zoom levels could be more flexible.

Per https://leafletjs.com/examples/zoom-levels/ (Fractional zoom) it is possible to have much smaller zoom increments by using the props zoomDelta and zoomSnap.

zoomDelta controls how much the view zooms in when using the buttons, while
zoomSnapwill snap the zoom to a nearest accepted value.

Perhaps add these as props with a default:
zoomDelta : 0.25
zoomSnap: 0 ?

[thor85]

wheelPxPerZoomLevel controls zoom delta when using mouse wheel. Higher value is same as lower ZoomDelta. Set it to a high value and it will use zoomSnap.

[anders-kiaer]

Moved to #110 for better overview. Closing this.