CXXGraph::Node::getData() should not be const

[nolankramer]

The current function signature of CXXGraph::Node::getData() is:

template <typename T>
const T &Node<T>::getData() const {
  return data;
}

This prevents users from modifying their stored data without const_cast. We should remove the qualifiers from this function. If the user wants read-only data, they should make the entire graph const.

[ZigRazor]

Maybe is better to duplicate the function, one with const on return value and on object, and one non-const function