Fix build on systems where Pythia is not available

The build process was broken on systems that did not have Pythia8.

The vertical_EAS example requires Pythia however the current CMake build process would still try to compile the example code even when Pythia was unavailable (resulting in a compile error). This MR explicitly adds the dependence on Pythia8 into the CMake configuration for the vertical air shower example (using an existing if (Pythia8-FOUND)-block). This has been tested on systems with and without Pythia.

The README suggests that Pythia is an optional component - is the plan for Pythia to remain optional? If so, it would probably be good to create at least one CI task to build CORSIKA8 on a system without Pythia.

Documentation/Examples/CMakeLists.txt

@@ -85,33 +85,34 @@ if (Pythia8_FOUND)
     CORSIKAprocesssequence
     )
   install (TARGETS cascade_proton_example DESTINATION share/examples)
-endif()
 
-add_executable(vertical_EAS vertical_EAS.cc)
-target_link_libraries (vertical_EAS
-  SuperStupidStack
-  CORSIKAunits
-  CORSIKAlogging
-  CORSIKArandom
-  ProcessSibyll
-  ProcessPythia
-  ProcessUrQMD
-  ProcessSwitch
-  CORSIKAcascade
-  ProcessEnergyLoss
-  ProcessObservationPlane
-  ProcessTrackingLine
-  ProcessParticleCut
-  ProcessStackInspector
-  ProcessInteractionCounter
-  CORSIKAprocesses
-  CORSIKAcascade
-  CORSIKAparticles
-  CORSIKAgeometry
-  CORSIKAenvironment
-  CORSIKAprocesssequence
-  )
-install (TARGETS vertical_EAS DESTINATION share/examples)
+  add_executable(vertical_EAS vertical_EAS.cc)
+  target_link_libraries (vertical_EAS
+    SuperStupidStack
+    CORSIKAunits
+    CORSIKAlogging
+    CORSIKArandom
+    ProcessSibyll
+    ProcessPythia
+    ProcessUrQMD
+    ProcessSwitch
+    CORSIKAcascade
+    ProcessEnergyLoss
+    ProcessObservationPlane
+    ProcessTrackingLine
+    ProcessParticleCut
+    ProcessStackInspector
+    ProcessInteractionCounter
+    CORSIKAprocesses
+    CORSIKAcascade
+    CORSIKAparticles
+    CORSIKAgeometry
+    CORSIKAenvironment
+    CORSIKAprocesssequence
+    )
+  install (TARGETS vertical_EAS DESTINATION share/examples)
+  
+endif()
 
 CORSIKA_ADD_TEST(stopping_power)
 target_link_libraries (stopping_power
@@ -123,7 +124,6 @@ target_link_libraries (stopping_power
   CORSIKAenvironment
   )
 
-install (TARGETS vertical_EAS DESTINATION share/examples)
 CORSIKA_ADD_TEST (staticsequence_example)
 target_link_libraries (staticsequence_example
   CORSIKAprocesssequence