FairModule::AddSensitiveVolume searches volume set twice

[dennisklein]

386912e introduced a double linear search of the volume on vList in FairModule::AddSensitiveVolume:

1. search happens here
2. search happens here, see FairVolumeList::addVolume

Possible fix involves changing the error behaviour of FairVolumeList::addVolume to return the failure to add to the caller, e.g. by returning a pointer to the added volume (which is nullptr in case of not adding it).