correct barn literals: _Xbarn -> _Xb

See merge request !97

Delete DiscreteProcess.h

See merge request !96

Resolve "hadronic interactions with pythia8"

Closes #149

See merge request !90

# Conflicts:
#   Documentation/Examples/cascade_example.cc
#   Processes/Pythia/testPythia.cc

Code and cmake cleanup

See merge request AirShowerPhysics/corsika!94

Tested manually in different containers to verify version compatibility.

Use -isystem to suppress warnings from headers

Changed to be in line with other processes

Failed on OS X

Improvements in Cascade and FlatExponential

See merge request AirShowerPhysics/corsika!93

Resolve "Exponential flat atmosphere"

Closes #151

See merge request !92

Resolve "Add energy loss process"

Closes #103

See merge request !83

Merge branch '103-add-energy-loss-process' of gitlab.ikp.kit.edu:AirShowerPhysics/corsika into 103-add-energy-loss-process