Merge branch '123-require-python-3-in-cmake' into 'master'

Resolve "Require python 3 in cmake"

Closes #123

See merge request AirShowerPhysics/corsika!77

Framework/Particles/CMakeLists.txt

+set(Python_ADDITIONAL_VERSIONS 3)
+find_package(PythonInterp 3 REQUIRED)
 
 add_custom_command (
   OUTPUT  ${PROJECT_BINARY_DIR}/Framework/Particles/GeneratedParticleProperties.inc

Processes/Sibyll/CMakeLists.txt

+set(Python_ADDITIONAL_VERSIONS 3)
+find_package(PythonInterp 3 REQUIRED)
+
 add_custom_command (
   OUTPUT  ${PROJECT_BINARY_DIR}/Processes/Sibyll/Generated.inc
   COMMAND ${PROJECT_SOURCE_DIR}/Processes/Sibyll/code_generator.py