Edf avoid duplications

TODO

• add options to the EDFConfig and link it with the nxtomomill edf2nx application. Those options are advanced options
  • duplicate_data: True by default. If false then one hdf external dataset will be created per frame and one hdf virtual will connect them all
  • external_link_path users can ask to have relative or absolute paths
• add test
• test reconstruction from nabu with it

notes

no performance test have been made on it. But we could not expect to have good performances with such a system (virtual dataset + one exteral dataset per frame). But this is the most direct way to add this feature.