imginfo system tests fail on negative rotation direction

`test_nexgen` fails because imginfo issues warnings as it expects increasing omega values and cannot identify the rotation axis.

The validity of these warnings and whether this could be considered a bug in imginfo depends no whether the downstream autoproc pipeline finds this to be an issue.

Our current strategy is just to try this at the next opportunity and see if autoproc gives us issues, and see if global phasing will fix any issues.

## Acceptance Criteria
- either imginfo / autoproc is fixed or we have to determine some other solution in the nexus file

Start by running the failing `test_nexgen` system test and inspect the warnings from imginfo about decreasing omega values and the rotation axis. Check whether the downstream autoproc pipeline is affected and whether global phasing changes the result. Done means fixing imginfo or autoproc, or documenting another solution in the nexus file.

Written by the indexing model from the issue text.