Spdhg unit tests - speed up - #1500

Describe your changes

Reduced image dimension, max iterations and update objective interval in order to speed up the Spdhg unit tests. Changing to GPU didn't seem to have any speed up locally.

Describe any testing you have performed

Please add any demo scripts to CIL-Demos/misc/

Link relevant issues

#1496

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License.
• I confirm that the contribution does not violate any intellectual property rights of third parties