Update cmake files in the examples related to HepMC and event generators

[andriish]

This PR is the first step to add HepMC3-based examples to Geant4. It concentrates mostly on the improvements to the currently used cmake build system for the current examples (i.e. HepMC2-based examples).

Please squash the commits.

In this PR:

• Updated FindPythia6.cmake
• Updated FindPythia8.cmake
• Added FindLHAPDF.cmake
• Added dummy sources needed to compile an event generator from pythia6 library
• Adjusted the CMakeLists.txt files in the examples to search properly for LHAPDF
• Adjusted the CMakeLists.txt files in the examples allow for any Fortran compiller, not only gfortran

The PR also includes a simple CI, which tests the compilation of the examples. The CI should be removed before the merge.

Tag @jrmadsen @drbenmorgan . It would be nice to tag Witold here, but I cannot find him in the list of members. Will have to write a mail.

Best regards,

Andrii