Pointer cleanup (#242)

- Made greater use of smart pointers, eliminating many instances of manual memory management
- Do not use pointers at all for many non-polymorphic classes
- Assorted other code cleanup

model/Variables.h

@@ -46,13 +46,13 @@ namespace glabels
 			/////////////////////////////////
 		public:
 			Variables() = default;
-			Variables( const Variables* variables );
+			Variables( const Variables& other );
 
 
 			/////////////////////////////////
-			// Object duplication
+			// Copy from other object
 			/////////////////////////////////
-			Variables* clone() const;
+			void copy( const Variables& other );
 
 
 			/////////////////////////////////