twa::~twa should call unregister_all_my_variables()

On Wed, Jan 6, 2016 at 7:34 PM, Miriam Blank sent the following question:

This code prints errors. How can I avoid them?

code:

    spot::bdd_dict_ptr dict = spot::make_bdd_dict();
    spot::kripke_ptr model = std::make_shared<MyKripke>(dict);
    model->register_ap("p");

errors:

    some maps are not empty
    Variable Map:
    0 Var[p] x1 { 0xb32fd0 }
    1 Free
    Anonymous lists: [0]
    Free list: (1, 1)
    Aborted

The current solution to put get_dict()->unregister_all_my_variables(this); in the destructor of MyKripke is not very elegant.

Since we introduced twa::register_ap() to hide most of the interactions with the bdd_dict, it should probably be cleaner if unregister_all_my_variables() was called in the twa destructor so that it is not necessary to call it in the destructor of every subclass.