There were some constructors in the DTOs which allowed construction of
DTO which weren't fully initialized. These constructors have been made
private, as they are still used in the 'fromJson' methods. Additionally,
a constructor with all required parameters to fully initialize the
class has been added.
Additionally, the Loader class has been modified, since it no longer can
assume it is able to default construct the parameter type. The parameter
is now stored as an optional.
Closes #15