Fix group removal
Fix group removal

When a layer group is removed, first try to remove the group
counterparts. If unsucessful, try to remove each of the group children.

thhomas/ol3-cesium,openlayers/ol3-cesium,thhomas/ol3-cesium,openlayers/ol3-cesium,openlayers/ol-cesium,oterral/ol3-cesium,openlayers/ol-cesium,thhomas/ol3-cesium,oterral/ol3-cesium