DUNE/Tasks: added EntityUtils.{hpp,cpp} for entity related global functions.
DUNE/Tasks: PlainEntity.cpp include cleanup.
DUNE/Tasks: replaced reserveEntityObject with templated reserveEntity, which returns a pointer to the internally created and stored object.

All Entity objects are now stored in the Task base class, which also takes care of memory management. Derived classes use the returned pointers to access them.
Changed tasks using reserveEntityObject to use the templated reserveEntity function instead.
DUNE/Tasks: leave the StatefulEntity state initialization to the class constructor.
Renamed PlainEntity->BasicEntity for naming consistency with other code.