This is an abstract role which provides an alternate constructor for
creating objects using parameters passed in from a configuration file. The
actual implementation of reading the configuration file is left to
concrete subroles.
It declares an attribute "configfile" and a class method
"new_with_config", and requires that concrete roles derived from it
implement the class method "get_config_from_file".
Attributes specified directly as arguments to "new_with_config" supercede
those in the configfile.