Sort image patches by similarity

[mzur]

Implement a method to sort thumbnails of images/videos in the volume overview by "similarity". This is comparable to the existing feature to sort by color. A simple measure for similarity would be to compute a PCA and sort by the value of the first component of each thumbnail. The sorting sequence can be computed when a volume is created and whenever the volume files change.

[mzur]

An experimental version of this feature has been implemented in #352. The development can continue in the similarity-sort branch.

[mzur]

We have determined that general sorting by "similarity" is 1. very hard to achieve algorithmically and 2. not automatically the best solution for the actual problem we want to solve. We agreed that the three different applications (volume overview, Largo, MAIA) probably need three different solutions. These will be discussed in the respective issues.

For the volume overview, the color sort feature might already solve most of the cases. We have to think abouth whether we actually need anything else.

[mzur]

Closing this for now. IMO sorting makes sense for MAIA and Largo but not for the volume overview.