[FIX,PYOPENMS] fix pyOpenMS after #875

- fromUnmodifiedString function was removed
[FIX] last fix
[MOVE] move pyOpenMS files to src
[FIX] fix peak boundary extension

- no extension of peak boundaries if zero value is encountered
- analogous to #880
[TEST] add test for zero cutoff
Merge pull request #904 from hroest/feature/fixPeakBoundaries

Feature/fix peak boundaries
[FIX] fix pyOpenMS files
Merge pull request #902 from hroest/fix/pyOpenMS_fix

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