Merge branch '263-pythia-8-crash-during-decay-of-particles-dsminus-and-particles-dsplus' into 'master'

Resolve "Pythia 8 crash during decay of particles::DsMinus and particles::DsPlus"

Closes #263

See merge request !194

Fix build on systems where Pythia is not available

See merge request !197

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.

Resolve "Improve merge request approval process"

Closes #260

See merge request !188

Try to manually find Pythia8 if `pythia8-config` is not found.

See merge request !198

On recent versions of Fedora and CentOS, the pythia8-devel packages
provided by the OS do not provide `pythia8-config` which causes the
current FindPythia method to fail. If `pythia8-config` is not found,
FindPythia now attempts to manually find Pythia8 in the standard
install directories.

Interaction histogram

See merge request !189

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

Resolve "Problem in move to sibyll2.3d"

Closes #267

See merge request AirShowerPhysics/corsika!196

Resolve "Update SIBYLL to 2.3d"

Closes #264

See merge request AirShowerPhysics/corsika!195

Resolve "README.md: checkout via HTTPS instead of ssh"

Closes #265

See merge request AirShowerPhysics/corsika!193