made CPDecimalNumber a subclass of CPNumber

Foundation/CPDecimalNumber.j

@@ -158,11 +158,18 @@ _cappdefaultDcmHandler = [CPDecimalNumberHandler decimalNumberHandlerWithRoundin
     mathematical operations to go with it.
     This class is mutable.
 */
-@implementation CPDecimalNumber : CPObject
+
+
+@implementation CPDecimalNumber : CPNumber
 {
     CPDecimal _data;
 }
 
++ (id)alloc
+{
+    return class_createInstance(self);
+}
+
 // initializers
 - (id)init
 {