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).
make this and all other accessors const