WIP: First version of FunctionalMDFWriter

This is genuine WIP and more meant to highlight some features.

1. HltDecReportsWriter is not functional, it gets the raw event and DecReports via get and then manipulates DAQ/RawEvent in place.
2. Added functional HltDecReportsEncoder which returns the DecReports in serialized format. Since RawBank has no destructor, it was not possible to put a RawBank onto the TES.
3. Added FunctionalMDFWriter which takes the RawEvent and the DecReports "raw bank" as input and writes the output to a file. The algorithm is a copy of the necessary parts of MDFWriter and MDFIO, no attempt to optimize it has been made.
4. As the input RawEvent is const, a copy of the RawEvent is created. The individual RawBanks are not copied, only the pointer to them. The DecReports raw bank is added.
5. To allow for multi-threaded writing a mutex is added to the execute of MDFWriter and FunctionalMDFWriter.

(The functions are copied and not inherited from the base class as the base class functions are not const.)

See also #5