Switch to positron base map

Created by: pmather

Since the change of policy by MapQuest about use of its base maps that took effect on 2016-07-11 (http://devblog.mapquest.com/2016/06/15/modernization-of-mapquest-result s-in-changes-to-open-tile-access/) that means we no longer get easy access to tile data, we switch for now to another base map provided by GeoBlacklight: positron.