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/nanomsg/FairMQSocketNN.cxx

@@ -20,7 +20,7 @@
 
 using namespace std;
 
-FairMQSocketNN::FairMQSocketNN(const string& type, const std::string& name, int numIoThreads)
+FairMQSocketNN::FairMQSocketNN(const string& type, const std::string& name, const int numIoThreads)
     : FairMQSocket(0, 0, NN_DONTWAIT)
     , fSocket(-1)
     , fId()