The BoundedWildcardDemo.java program 
uses the other four Java class files.

Bounded wildcards are particularly
important when you are creating a
generic type that will operate on a
class hierarchy.

Here's the general structure of this example:
- TwoD is a two-dimensional co-ordinate class
- ThreeD is a three-dimensional co-ordinate
  class that inherits from TwoD
- FourD is a four-dimensional co-ordinate
  class that inherits from ThreeD
- Coords is a class that holds an array of
  co-ordinate values that can be objects of
  any of the other three classes
  
Note:
For simplicity the instance variables in these
very simple classes have not been given the usual
"private" access qualifier so that we don't need
to have any getters when we're outputting the values.