Unhide counter from mc_trks
In https://git.km3net.de/common/km3net-dataformat/-/issues/93 the counter field was added to Trk which is used in MC tracks. This needs to be unhidden from mc_tracks, which is done in this MR.
In https://git.km3net.de/common/km3net-dataformat/-/issues/93 the counter field was added to Trk which is used in MC tracks. This needs to be unhidden from mc_tracks, which is done in this MR.