Improvements for external LHE files

Changes introduced

• Skip nevents check for MG in logParser if external LHE files are used
• Automatically skip athena running in CI if external LHE files are used (technically the log.generate.short file will be deleted)

Tests

Based on ATLMCPROD-8589

Before changes

RuntimeError: nevents was not set in the MG5aMC jO

After changes

Notice that the log.generate.short file has been deleted and is thus not listed in the list of files to be committed.

logParser gives

---------------------
Generate transform params:
---------------------
- ecmEnergy =  13000
WARNING: evgenConfig.nEventsPerJob is not defined in the jO. Will set to default=10000
- nEventsPerJob =  10000
- Requested output events =  5000
- transform =  Gen_tf
- inputFilesPerJob =  0   	
- inputGeneratorFile =  MGaMCPy8EG_A14_NNPDF_ggH_FxFx_012jNLO__500.events
...
---------------------
 Summary:
---------------------
Errors : 0 , Warnings : 3  -> Some warnings encountered, check that these are ok before submitting production!

Notice the new printout inputGeneratorFile which is used to steer the other new features in this commit.

Issues resolved

Closes #