Gmr module: Add function ToModel in gmt_color structure.

modules/gmt/modgmt_structs.h

@@ -565,6 +565,17 @@ struct gmt_color: public gmt_struct
   cyan=cc.cyan; magenta=cc.magenta; yellow=cc.yellow; black=cc.black;
  }
 
+ void ToModel(ColorModel newmodel)
+ {
+  switch(newmodel)
+  {
+   case(RGB):  ToRGB();  return;
+   case(GRAY): ToGray(); return;
+   case(HSV):  ToHSV();  return;
+   case(CMYK): ToCMYK(); return;
+  }
+ }
+
  // Interpret one numeric argument as gray value
  bool Convert(double gr, std::string& err)
  {