The update concludes the discussion in the CWG4 meeting

Introducing template parameters for the header message and the payload message.
Correcting some typos.

The test is to first extend a compilation check and can be later
extended to a unit test

Creating new module 'DataFormats' on top level. The recent development
regarding data formats goes into DataFormats/Generic.

The original header definition has served as basis for the discussion,
the final definition has been developed elsewhere and is soon to be
merged. The very early definition is temporarily kept for the unit test.

When using clang, writing to output stream caused segmentation violation
because the stream object was not returned by the operator which has been
used just before.
Adding also some other return statements (though irrelevant for the crash).

Crash log
* thread #1: tid = 0x597d9, 0x0000000100001f7f testMessageList`void print_list<AliceO2::Format::messageList<TestMsg, SimpleHeader_t> >(list=<unavailable>, hdrsel=<unavailable>) + 191 at testMessageList.cxx:77, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
    frame #0: 0x0000000100001f7f testMessageList`void print_list<AliceO2::Format::messageList<TestMsg, SimpleHeader_t> >(list=<unavailable>, hdrsel=<unavailable>) + 191 at testMessageList.cxx:77
   74          it != list.end();
   75          ++it) {
   76       // the iterator defines a conversion operator to the header type
-> 77       std::cout << static_cast<typename ListType::header_type>(it) << std::end;
   78       // dereferencing of the iterator gives the payload
   79       std::cout << *it << std::end;
   80     }