New addTo function (standard in Leaflet)

Additionally to the addLayerTo function, an addTo function has been
added. The addTo function is the standard function in other Leaflet
layers for adding the layer to the map.
Added option to deactivate point filter

Points to be drawn are automatically filtered using the QuadTree. For
drawing curves, simple filtering by points does not work, hence the
option to deactivate this filtering. This is certainly not an elegant
solution, but it is probably still ok for many situations. Need to
introduce bounds checking based on geometry bounding boxes or similar
(again a simple check won’t work for some curves).