Changes in the CMake part to make it work for the LCG stack
These are a number of small changes in the CMake part that are needed to add Garfield++ into the LCG software stack
- The header guards HEP_SYSTEM_OF_UNITS_H/HEP_PHYSICAL_CONSTANTS_H clashed with the CLHEP ones
- Installation of examples under the /examples directory instead of /bin
- Reduced the list of ROOT libraries to be linked to the minimal ones
- Make use of the variable CMAKE_CXX_STANDARD to set the C++ standard instead of compilation flags
- Install header files under
include/Garfield