[MED-13][external] nifty export from multi slot item

[NooshinGhavami]

Problem

At the moment nifti export only outputs single slot even for multi-slot items in the workview. We want the output to be modified so that annotations are exported per slot

Solution

Modifications were made to the darwin/exporter/nifti.py to handle these modifications and tests/darwin/exporter/export_nifti_test.py new unit tests were added.

Changelog

Modifying code and adding new functionality to be able to handle multi-slot exports for nifti images.

[linear]

MED-13 Nifti export from multi-slot item

Description:

We currently support nifti exports from our MPR view as well as single volumes. However we don't support export to nifti for complex multi-slot items which might have multiple volumes.

This is needed for Jubilant a 50k prospect.

Acceptance Criteria:

[owencjones]

@NooshinGhavami have converted to draft whilst these tests are failing. If you need anything reach out on slack.

[NooshinGhavami]

@owencjones tests should be fixed now! Thank you.

[owencjones]

I like it, it's really made the flow clearer and more readable, although anything with complex affine geometry is never going to be easy to process mentally at first!

I've made loads of comments, but mostly down to your judgement, the only ones I'd like to require are the type changes.

[owencjones]

A few of your functions are still missing return types, but we can ticket that for further improvement later, it's already a nice step forward. Good stuff.

[tomvars]

Looks good to me! We've tested this for a variety of output configurations. It should make its way into our platform for exports next week.