Handle correctly "strange" gene datasets

`Vip/1103.npy` contains only one gene slice.

To Do:

- [ ] Consider warning the user once a `GeneVolume` is instantiated with this kind of dataset (empty or very small dataset - maybe we could also just output some metadata of the gene dataset).
- [ ] Prevent any problem of computation in `GeneInterpolate` and `GeneOpticalFlow`

Start with the `Vip/1103.npy` dataset and inspect how `GeneVolume` handles empty or single-slice inputs before tracing those inputs through `GeneInterpolate` and `GeneOpticalFlow`. Done means the dataset condition is reported appropriately and neither computation fails on empty or very small gene datasets.

Written by the indexing model from the issue text.