Reconsider using `nrrd` in `DVC`

Currently, some DVC stages (for example `combine-ccfv2-fiber`) are creating `NRRD` files as output. However, the `pynrrd` automatically inserts a header containing the time stamp which makes the process non reproducible. `md5` is then changing at each iteration.

Start by examining the DVC stages, including combine-ccfv2-fiber, and the code path that uses pynrrd to create NRRD outputs. Compare generated headers and md5 values across iterations to understand the reproducibility issue. Done should mean the affected stages produce stable outputs, or the issue has a documented decision on replacing NRRD.

Written by the indexing model from the issue text.