[FIX] allow empty RT input

- OpenSwathWorkflow allows to not specify an iRT input file (it will
  simply assume that no iRT correction is requested and print a warning)
[FIX] move computation of overlapping features

- perform computation of overlapping features _after_ the boundaries are
  re-computed
[DEBUG] add option to get irt chromatograms for debugging
Merge pull request #781 from hroest/fix/OpenSwathFixes

Fix/open swath fixes