add isBaseLayer() function to Layer

git-svn-id: http://svn.openlayers.org/trunk/openlayers@622 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer.js

@@ -56,6 +56,15 @@ OpenLayers.Layer.prototype = {
     setMap: function(map) {
         this.map = map;
     },
+  
+    /**
+     * @returns Whether or not the layer is a base layer. This should be 
+     *          determined individually by all subclasses. 
+     * @type Boolean
+     */
+    isBaseLayer: function() {
+       //this function should be implemented by all subclasses.
+    },
     
     /**
     * @returns Whether or not the layer is visible