* src/graph/graph.hh (boxed_label): If State_Data==void, inherit from
std::tuple<> and implement a data() method.
(digraph::state_data): Return by reference.
* src/graphtest/ngraph.cc, src/graphtest/ngraph.test: Test the
case where State_Data implements the spot::state interface.
* src/graph/ngraph.hh: New file.
* src/graph/Makefile.am: Add it.
* src/graphtest/ngraph.cc, src/graphtest/ngraph.test: New files.
* src/graphtest/Makefile.am: Add them