Constructed Classes need descriptions

[jsrehak]

Many class interfaces need a std::string description() const method that returns a string describing the class. Any class that is the default implementation should have (default) at the beginning of the string.

[jsrehak]

A new utility class has been added bart::utility::HasDescription that has a description() method, that can be set using set_description(std::string). Default implementations can be set using is_default_implementation() that will add a (default) marker at the start of the string returned by description().