loadable_aut: introduce LoadableAut and use it for BDDBUTreeAut

The new class template

  template <class TBaseAut> VATA::LoadableAut;

can be used to extend the string loading functionality of automata
classes. The automaton class need to provide a protected (or public)
method template with the following signature

  template <
    class StateTranslFunc,
    class SymbolTranslFunc>
  void loadFromAutDescInternal(
    const AutDescription&       desc,
    StateTranslFunc             stateTransl,
    SymbolTranslFunc            symbolTransl,
    const std::string&          params = "");

and LoadableAut extends it with the following methods

  void LoadFromString(parser, str, params);
  void LoadFromString(parser, str, stateDict, params);
  void LoadFromString(parser, str, stateTransl, params);

  void LoadFromAutDesc(desc, params);
  void LoadFromAutDesc(desc, stateDict, params);
  void LoadFromAutDesc(desc, stateTransl, params);
loadable_aut: change BDDTDTreeAut and ExplicitTreeAut to use the template
loadable_aut: add DumpToString() methods