Introducing Resources, a dynamic wrapper around the object that contains the resources.
Now the activities are dynamically typed.
Some small refactorings.
Now copying works for activites with multiple resources.
Activity.fillFromDB actually does the job of the constructor -> turn it into the constructor.
Copying must not assign typed objects, only their state internals, to the surrounding object's state internals.
Split test file in two parts.
Activity can list all participants of all resources -> now we can send mail to all participants again :-) Index is also simplified.