add package index to the MM variables of the NSWTP

This MR adds a variable linking the payload and the index of the MM packets in the sTGC trigger processor decoding. This will change the output of Run3 data reconstruction but only affects the NSW trigger processor container which is only used for NSW trigger performance studies. And since this type of data is not present in the BS file used for the CI test it is not flagged as output changing.

cc @ekajomov @estel @sfrances