Draft: Stitching

• add an option to avoid duplicating data for HDF5
• make a first prototype avoiding copy (but with plenty of Virtual Source)
  • do the prototype
  • rework to simplify understanding ? Get a PostProcessing class and a PostProcessingNC class or some 'dump to file' helper ?
  • test frame flip correctly handled
• make a fast-stitcher version which 'by pass' the reading of each frame and which only compute the stitching and then do the virtual dataset creation
• fix issue about axis on the stitched volume (#487) / add an option to define the output stitching axis