Investigate section numbers of gene datasets

`Vip/1102.json` has twice the same section number in his section_numbers list.

To Do:
- [ ] Investigate if there is something wrong with the computation of the section numbers
- [ ] Consider warning the user once a `GeneVolume` is instantiated with this kind of data
- [ ] Prevent any problem of computation in `GeneInterpolate` and `GeneOpticalFlow`

Start with Vip/1102.json and trace how its section_numbers list is computed when a GeneVolume is instantiated. Then inspect the GeneInterpolate and GeneOpticalFlow entry points for assumptions about unique section numbers; done means the source of the duplicate is understood, users can be warned about invalid data, and both computations are protected from the problem.

Written by the indexing model from the issue text.