Add a block diagonal operator

[MargaretDuff]

Changes

The diagonal operator checks if a block data container is passed to it and then either goes to the old diagonal operator or a new one, which deals with the block operations (without ZeroOperators).

Written with the help of @bryceshirley

To do for merging into CIL

• Add unit tests including when you have blocks of blocks
• Check the documentation (it was written in a rush)

Testing you performed

Please add any demo scripts to https://github.com/TomographicImaging/CIL-Demos/tree/main/misc

Related issues/links

Reopens a very old issue #1568

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

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