#15 Add suport for tempalted Groups

[Hernan Saez]

The Group class is designed to hold any number of Node instances but, even when this design is correct most of the time, the developer is force to perform dynamic casts to retrieve the actual node type under certain circumstances. Considering that in all of these cases the actual node type is known at compile time, it would be a good addition to have a templated version of the Group class that can store specific nodes. This will avoid the runtime casts while restricting the stored nodes type to a known one.