gazebo_imu: ADDITIONAL_WRAPPER group should be not mandatory

[Nicogene]

The device gazebo_imu can be opened and attached to the "old" wrapper (see http://yarp.it/git-master/classServerInertial.html) and to the "new" one (see http://yarp.it/git-master/classMultipleAnalogSensorsServer.html) with the WRAPPER and ADDITIONAL_WRAPPER configuration logic.

Unfortunately, both wrappers are mandatory in the configuration phase, since not specifying the ADDITIONAL_WRAPPER you get this error:

[ERROR] GazeboYarpIMU : [ADDITIONAL_WRAPPER] group not found in config file

I think it could be better if this would be an optional parameter

[traversaro]

Related to #534 . Since #532, no network wrapper should be mandatory.

[traversaro]

Fixed by #586 .