zenodo_get dependency and unit test errors

[paskino]

Description

I wonder whether some of our "dependencies" are actually dependencies. For instance zenodo_get is a new dependency but one may be using CIL without requiring it. Some unit test currently fail if that's not installed. Wouldn't it be more suitable to skip them?

https://github.com/SyneRBI/SIRF-SuperBuild/actions/runs/13288866230/job/37104246990?pr=937

I guess this requires a wider discussion about requirements and optional dependencies.