change in register_names protocol

move to returning the attribute names
getting and setting can then be automated in the base class

lib/risc/instructions/label.rb

@@ -24,7 +24,7 @@ module Risc
     attr_reader :name , :address
 
     # return an array of names of registers that is used by the instruction
-    def register_names
+    def register_attributes
       []
     end