title: Calculating zoomToSpan() degrees based on min/max lat/lng to be in the mapView
body: <p>I have a cluster of items on an overlay in a mapView.<br>
  I know where the top/bottom/left/right most objects are.<br>
  I want to use the zoomToSpan() method to zoom onto that area.  </p>
  
  <p>What is the proper way to calculate the lat/lng degrees for this method ?</p>