LHE file filtering: handling the case of outputTXTFile
requested to merge atlas-physics/pmg/mcexperts/powheg-experts/athena:21.6-UpdateLHEFilters into 21.6
I modified the lhe file filtering in order to allow the use of --outputTXTFile. The use-case is when we need to produce the lhe file on-the-fly, filter it, and save it after the filter. I tested that it works (using Powheg), whether or not we shower it afterwards.