stitching: masive refactoring

the goal is to:

• ease visibility and understanding of the code source
• separate saving of the stitching. Especially to smooth no data duplication when possible
• allow stitching along all the dimensions (but still along a single axis)
  • add y_stitching module and function to handle something like tetra-acqui (proto but will validate the refactoring)
• avoid data duplication. warning: to avoid data duplication none of the frame must be flipped. This come from the fact that we cannot define h5py VirtualSource with another reading direction (negative step) in order to absorb those flip. And then we will need to duplicate data again...