New: GetSelectedReading to replace functionality of ReadMeiApp

 -Added GetSelectedReading function to select the user's chosen <rdg> element or the default if none is chosen.

-Added code to ReadMeiLayerElement to allow <app>/<rdg> elements as children of layers ( http://digitalduchemin.org/mei/DC1022.xml for testing )

-Added prototype code to replace ReadMeIApp in ReadScoreBasedMei but did not enable yet.