Fix warnings

#54 (closed)

Done:

• No warnings left.
• I also added a virtual destructor to the iterator class, since it was missing and the memory of the two pointers from the child classes was lost.
• Limited some of the iterator internal stuff to private.

TODO?

• I was looking at the child iterator classes and I don't understand why the ptr_ members are there. We have the data pointer and the index and that should be everything that we need, so I assume that ptr_ could be removed still.
• The access operators currently allow changing the values in the buffer. Should that be the case? Currently a reference is returned (instead of const reference). So basically I would say we need a "const_iterator" type of object.