Fix for bug introduced by 95ff71c8 (Seedclusters were not assigned properly)

95ff71c8 fixed the Seedcluster assignment in GblTrack for tracks without clusters in the first planes. Track fitting is done relative to this cluster, while the result is still calculated relative to the first cluster in clusters, which must not necessarily be the same.

This is fixed by adding the seedcluster to the membervariables of GblTrack to ensure correct calculation also after the Track object is exported to a root file.