Unpacker for phase-2 outer tracker

This pull request introduces the code required to unpack data coming from the prototypes of phase-2 tracker 2S modules, and is designed to evolve towards the code used for future data-taking with CMS. It was tested with actual data collected in December in DESY, and presented at the tracker simulation upgrade meeting on June 17: https://indico.cern.ch/event/305424/session/3/contribution/4/material/slides/0.pdf

While it touches several packages, there is basically no modification of existing code. These are new classes designed to live without interfering with the existing code. Additions are for the unpacker itself (in EventFilter), for the digi data format (in DataFormat) and for the cabling (CondFormat and related).
- the name will collide with EventFilter/SiStripRawToDigi/interface/SiStripRawToDigiUnpacker.h (the namespace and the class are the same). This conflict should be resolved.
- How specific is this to the planned Phase2Tracker (what we have in the TP at least, PS and 2S modules etc)?
If this will not be used other than for this prototype in beam tests, maybe call it this way
- the header file name should match the class name