[FIX,PYOPENMS] fix pyOpenMS after #875

- fromUnmodifiedString function was removed
[FIX] last fix
[MOVE] move pyOpenMS files to src
[FIX] fix pyOpenMS files
Merge pull request #902 from hroest/fix/pyOpenMS_fix

[FIX,PYOPENMS] fix pyOpenMS after #875, #896