Guard against more than one track_scoring object in egsinp

[mxxo]

As discussed in #745, ensure that multiple EGS_TrackScoring objects are not defined as this can lead to bugs at write time when the objects try to write to the same file (i.e. if the file name addition is blank). More than one track scoring object is not seen as a required feature, since photons, electrons and positrons can all be toggled in egs_view.

:start ausgab object:
    library = egs_track_scoring
    name    = some_name
    score photons   = yes or no # optional, yes assumed if missing
    score electrons = yes or no # optional, yes assumed if missing
    score positrons = yes or no # optional, yes assumed if missing
    start scoring   = event_number # optional, 0 assumed if missing
    stop  scoring   = event_number # optional, 1024 assumed if missing
    buffer size     = size         # optional, 1024 assumed if missing
    file name addition = some_string # optional, empty string assumed if missing
:stop ausgab object:

[rtownson]

@mainegra is there a debugging feature here where you might output tracks in limited "start scoring/stop scoring" ranges that would make it useful to have multiple track ausgab objects? I had forgotten 'file name addition' was an option - maybe we just need to defend against them having the same output file..

[mainegra]

Fellas, my understanding was that the possibility to modify the file name was just for that: having a ptrack file name different from the default name used based on the input file name. So I agree that one ptrack file should be enough.