add a map getUnits method, r=ahocevar (Closes 1591)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@7374 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Map.js

@@ -1763,6 +1763,21 @@ OpenLayers.Map = OpenLayers.Class({
         return resolution;
     },
 
+    /**
+     * APIMethod: getUnits
+     * 
+     * Returns:
+     * {Float} The current units of the map. 
+     *         If no baselayer is set, returns null.
+     */
+    getUnits: function () {
+        var units = null;
+        if (this.baseLayer != null) {
+            units = this.baseLayer.units;
+        }
+        return units;
+    },
+
      /**
       * APIMethod: getScale
       *