* reset_values function to set all Values to their defaul-construct state
* read-only access to all variables of some type via get_all<T>
* Variables cloning
* convenience functions: get_or_throw, get_value
Value subtypes now contain a value_type typedef and require a const value_type& get_value() const member function.

add a VariableAccesor class in Variables and corresponding create_accesor and get_fast functions in Variables, plus tests

Add PositionRef value class, add Position:: nowhere/begin/end, add SentenceContext get_[abs|rel]_position takint Position or Positionref
Add tests for sc's get_*_position