merge method
merge weighted lat long value the current WeightedLatLng,
Implementation
void merge(double x, double y, double intensity) {
var newX = (x * intensity + latLng.longitude * this.intensity) /
(intensity + this.intensity);
var newY = (y * intensity + latLng.latitude * this.intensity) /
(intensity + this.intensity);
latLng = LatLng(newY, newX);
this.intensity += intensity;
}