Added asDouble method to ZString.

ZString.h

@@ -128,6 +128,13 @@ namespace coreutils {
       
       int asInteger();
       
+      ///
+      /// Return the floating point  value of the ZString from the cursor to the first
+      /// non-numeric character.
+      ///
+      
+      double asDouble();
+      
       ///
       ///
       ///