- Add multipart support to the interface and enable its use out of tasks.

  Examples on the use out of tasks are provided in:
  `example/Tutorial3/digitization/TestDetectorDigiLoader.tpl:76-85`: sending a part.
  `example/Tutorial3/reconstruction/FairTestDetectorMQRecoTask.tpl:177-182`: receiving a part.

- This commit also makes structure within processorTask more consistent with samplerTask.

- Quick fix for Protobuf to get rid of C++11 requirement.