Improve roi

• Modifies ROI functioning:
  • Roi can be applied via origin or center
  • Size argument corresponds to whole size of the roi
  • Ceil size in case of odd size
• Adds standard buttons: Reset, Apply and Ok.
  • Apply sets the stack with the region of interest but does not modify the dataset data.
  • Ok replaces the dataset data with the data of the stack: if roi hasn't been applied with Apply, no roi is added.
  • Reset restores stack with data of the dataset.

Closes #8 (closed)