Gmt module: Add BaseM2Double for easy access to parameters with multiple names.

modules/gmt/modgmt_func.h

@@ -236,6 +236,9 @@ class SourceMultiInputNames
  bool Exist() const {return exist;}
 };
 
+// Simple type for multiple input names
+typedef GetDouble<SourceMultiInputNames<double> > BaseM2Double;
+
 // Definitions for ObjectGMTCoord
 template<> class TypeStorage<struct gmt_coord>: public gTypeStorage<struct gmt_coord,ObjectReal,ObjectInt,ObjectString,ObjectGMTCoord,ObjectList> {};
 typedef TypeStorage<struct gmt_coord>::Base2Type Base2Coord;