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).
Most of these should be defined in separate headers, unless the code has to be there all combined
and is very likely to be used whenever one of the classes is needed.
The two unpackers don't seem to show up anywhere.