Use "Setting nevents" to extract number of events for MG and add protection for multiple such printouts

Description of bug

Bug related to nevents extraction for MadGraph, affecting releases made after Zach's changes in !800 (merged) See #131 (closed)

Changes introduced

• Use

MadGraphUtils ... Setting nevents ... = ...

to extract nevents

• add warning if multiple "Setting nevents" printouts are found

Tests

Before test

After test

Also you if I put the "Setting nevents" line twice in the log file the new warning shows up:

Still works on old releases

Issues resolved

Closes #131 (closed)