Relation: new template class
Relation: new template class

Provide a generic object to wrap arguments for service operations
that work with relations between object (assign, unassign).

Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server