Improve missing shooting date strategy using a photo event based approach

[dacr]

Related to #20 :

• Add a secondary place deduction strategy based on events
  • assumption : most of the photos of a given event should have been made in a same "place"
  • take the barycentre ?

" by event, check all positions, if they are all in a "same" approximative location then missing positions can be deducted"