fixed InteractionHistogram::operator+()

See merge request AirShowerPhysics/corsika!203

Resolve "remove examples from coverage"

Closes #266

See merge request AirShowerPhysics/corsika!199

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 AirShowerPhysics/corsika!194

Fix build on systems where Pythia is not available

See merge request AirShowerPhysics/corsika!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 AirShowerPhysics/corsika!188

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

See merge request AirShowerPhysics/corsika!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 AirShowerPhysics/corsika!189