Feature #48 - Add group messages to the ONE simulator

s.t. it also works when not executed with gradle

*add check of host address range in constructor.
There might have been a problem when the group size is bigger
than the specified host range. So this is checked beforehand.

since we're only using static groups in which nodes are not
joining or leaving during simulation, we don't need nodes to
know, in which group the are in. So this feature was removed.

*create new abstract test class for message create events
*small fixes
*finish group test
*reorganize testsettings

* removed manual assignemnt using the settings file

*new interface addressable that is implemented by DTNHost and
Group
* remove unnecessary group leave

* add new group message class
* nodes are able to join groups