experimental data handle code

exploratory branch to try out a few data handle related items, aimed at supporting a 'vector of data handles'. This requires (amongst others) the possibility of 'renouncing' (the opposite of declaring) a handle as a handle goes out of scope (as eg. the vector of handles gets resized). Adding and renouncing handles should only be allowed up to the moment the data flow graph is build, ie. until 'initialise'. Once the visitor which determines the graph has run, any new attempt at declaring or renouncing data handles should be considered a (fatal) error -- until (at least) such time as that the framework knows that the graph should be re-generated.