Allow initial = None in SIRT

[MargaretDuff]

Changes

SIRT takes initial = None.

To maintain the order of arguments to SIRT, both operator and data have default None when the arguments are passed to the init but raise a ValueError if they are not provided.

Testing you performed

New unit test implemented

Related issues/links

Fixes #1895

Checklist

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have updated the relevant documentation
• 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

[jakobsj]

LGTM!

[MargaretDuff]

Jenkins is happy