[TEST,PYOPENMS] fix test after #754
[FEATURE,PYOPENMS] adopt MSChromatogram after MSSpectrum (#754)

adopt MSChromatogram after the changes in
97e6477e3d2b57881e651d17fc0bac7675bfddd5 to MSSpectrum
Merge pull request #798 from hroest/fix/pyOpenMS_testfix

Fix/py open ms testfix