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/test/push-pull/FairMQTestPull.cxx

@@ -23,9 +23,9 @@ FairMQTestPull::FairMQTestPull()
 
 void FairMQTestPull::Run()
 {
-    std::unique_ptr<FairMQMessage> msg(fTransportFactory->CreateMessage());
+    std::unique_ptr<FairMQMessage> msg(NewMessage());
 
-    if (fChannels.at("data").at(0).Receive(msg) >= 0)
+    if (Receive(msg, "data") >= 0)
     {
         LOG(INFO) << "PUSH-PULL test successfull";
     }