include proposal in build system
incl. proposal in cmake build. fix several problems.

The proposal library has to be installed manually and corresponding cmake variables have to be exported.

The `vertical_EAS` example requires Pythia however the build process
still tried to compile the example code even when Pythia was
unavailable. This adds an explicit dependence on Pythia for the
vertical air shower example.

make vertical_EAS non-test executable

modified vertical_EAS for interaction counting project

more bins on upper end

disable energy loss, read A,Z,E from argv

back to SIBYLL decays

add QGSJet

redesigned/refactored LuminosityCounter

const-correctness in Sibyll/NuclearInteraction

updated vertical_EAS to new LuminosityCounter design

comply to C++ standard

renamed to InteractionCounter