Broadcast message type

Ghost Userrequested to merge
feature-#40-broadcast-message-type into master

Adds a broadcast message class and modifies the original Message class and calling classes s.t. all classes we later want to use with broadcast or group messages do not depend on Message.getTo. Also enables MessageEventGenerator to create broadcast messages.

Documentation can be found at https://wikis.uni-paderborn.de/pgandi/Simulator_Code_Documentation#Message_Types.

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