Multipart

- 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.

- Make structure within processor task more consistent with sampler task.

- Add `WARN` option and MQLOG macro to FairMQLogger.