Support spatio-temporal flow fields

[aschuh-hf]

Currently, most flow field functions assume that the number of vector field channels is equal to the number of grid dimensions. However, in case of a spatio-temporal flow field, each vector field only deforms space.

The shape of a spatio-temporal flow field tensor is (N, D, T, ..., X), i.e.,

• (N, 2, T, Y, X) for a 2D+t flow field, and
• (N, 3, T, Z, Y, X) for a 3D+t flow field.

Also the Grid would only apply to the spatial dimensions. The spacing between time points along the temporal dimension may need to be stored in a separate metadata field.

At the moment, one may need to use a sequence / batch of 2D or 3D flow fields instead, but computing a temporal partial derivative is then also not supported by the respective image functions.