[FEATURE,TEST] mssim more ground truth in id xml

 * extended MSSim to export not only the peptide in idXML with the highest intensity contribution, but all scored by their individual contribution.
 * extended MSSim_test to actually test the composition of the generated data without testing ordering
 * [unrelated] made TOPPDocumenter more robust to errors
can you test for `ms_it->getPrecursors().empty` ? or how do you ensure this never happens?