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/FairMQTestPush.cxx

@@ -23,8 +23,8 @@ FairMQTestPush::FairMQTestPush()
 
 void FairMQTestPush::Run()
 {
-    std::unique_ptr<FairMQMessage> msg(fTransportFactory->CreateMessage());
-    fChannels.at("data").at(0).Send(msg);
+    std::unique_ptr<FairMQMessage> msg(NewMessage());
+    Send(msg, "data");
 }
 
 FairMQTestPush::~FairMQTestPush()