Tsv vis

Code needed for the new Volume Time Series visualizer.

- `get_rotated_volume_slice()` is the same as `get_volume_slice()` but it returns a matrix with its last dimension inverted, to avoid this same computation on the client's side.
- `get_volume_shape()` is an useful function for the Volume Time Series Visualizer. It allows the client to know how many time points compose the image, and not only its shape in space