FairMQ: Extend Multipart and messaging API

- Extend the multipart API to allow sending vectors of messages or helper
   thin wrapper FairMQParts. See example in examples/MQ/8-multipart.
 - NewMessage() can be used in devices instead of
   fTransportFactory->CreateMessage().
   Possible arguments remain unchanged (no args, size or data+size).
 - Send()/Receive() methods can be used in devices instead of
   fChannels.at("chan").at(i).Send()/Receive():
   Send(msg, "chan", i = 0), Receive(msg, "chan", i = 0).
 - Use the new methods in MQ examples and tests.
 - No breaking changes, but FAIRMQ_INTERFACE_VERSION is incremented to 3
   to allow to check for new methods.

fairmq/CMakeLists.txt

@@ -91,6 +91,7 @@ EndIf(NANOMSG_FOUND)
 # to copy src that are header-only files (e.g. c++ template) for FairRoot external installation
 # manual install (globbing add not recommended)
 Set(FAIRMQHEADERS
+  FairMQParts.h
   devices/GenericSampler.h
   devices/GenericSampler.tpl
   devices/GenericProcessor.h