Small fix

[geonove]

Small bug fix that I missed in #95

[leerho]

It would be real helpful if you could explain what the problem was that this fixes. :)
Thanks!

[geonove]

Change was to store the merge trial error into the pre-allocated rank_errors slot for the current trial instead of appending a new element. Previously push_back grew the inner vectors each trial and misaligned the rank/trial grid; now it writes to rank_errors[j++][t], matching the intended rank-by-trial structure (since it the rank error vector is pre-allocated).