implemented CombinedStack with unit tests

.gitlab-ci.yml

Documentation/Examples/cascade_example.cc

Framework/Particles/NuclearData.xml

Framework/Particles/ParticleProperties.cc

Framework/Particles/ParticleProperties.h

Framework/Particles/pdxml_reader.py

Framework/Particles/testParticles.cc

Framework/Units/PhysicalConstants.h

Framework/Units/PhysicalUnits.h

Processes/Sibyll/Interaction.h

Processes/Sibyll/NuclearInteraction.h

Processes/Sibyll/SibStack.h

Processes/Sibyll/testSibyll.cc

Processes/StackInspector/StackInspector.cc

Stack/NuclearStackExtension/NuclearStackExtension.h

Stack/SuperStupidStack/SuperStupidStack.h