message: adds Marcel Koch to contributors.txt
diff: @@ -15,6 +15,7 @@ Heroux Mike <maherou@sandia.gov> Sandia National Laboratories
  Hoemmen Mark <mhoemme@sandia.gov> Sandia National Laboratories
  Holeksa Claudius <mail@keldu.de> Karlsruhe Institute of Technology
  Kashi Aditya <aditya.kashi@kit.edu> Karlsruhe Institute of Technology
  +Koch Marcel <marcel.koch@kit.edu> Karlsruhe Institute of Technology
  Maier Matthias <matthias@43-1.org> Texas A&M University
  Nayak Pratik <pratik.nayak@kit.edu> Karlsruhe Institute of Technology
  Olenik Gregor <go@hpsim.de> HPSim
  