New GenericDetectors directory structure and Digi libs.

New GenericDetectors directory structure and Digi libs.

• Added new structure to GenericDetectors
• Building Separate digitization libraries.
• Started to follow "modern cmake" build style.
• The directory structure and library grouping is a work-in-progress.
• The DD4hep detectors (ie *_geo.cpp) and digi library sources are in the same directories but broken down by type (e.g. trackers, calorimeters, beamline, etc...)
• Ideally we will document every parameterized detector's usage, but where this documentation should live is not obvious.
• Currently I am thinking the documentation should be in a "doc" directory, but I am not sure this is ultimately best...
• Furthermore the compact examples to match probably should go with this too ...

Below is the directory structure envisioned (and halfway implemented)

`GenericDetectors
├── beamline      # beamline builds two libraries: detector and digitization
│   ├── cmake     # holds only xxxxConfig.cmake.in
│   ├── include
│   └── src
├── calorimeters  # calorimeters builds two libraries: detector and digitization
│   ├── cmake     # holds only xxxxConfig.cmake.in
│   ├── include
│   └── src
├── compact       # not sure this is the best place
└── trackers
    ├── cmake
    ├── compact   # this might be a better "compact" directory
    ├── doc
    ├── include
    └── src

Maybe we don't even want a "compact" directory and the xml files should just live in "doc" or "examples"