Remove any duplication of side output checking between UI layer and YAML checker?

Both the UI layer (`ui_layer.py`) and the YAML checker (`yaml_checker.py`) seem to have code mentioning the checking of validity of side output references. For example, the following two functions have very similar names:

UI layer: https://github.com/DiamondLightSource/httomo/blob/ae5610a83a691436e0e7deff3a71c2af3f89568e/httomo/ui_layer.py#L239-L261

YAML checker: https://github.com/DiamondLightSource/httomo/blob/ae5610a83a691436e0e7deff3a71c2af3f89568e/httomo/yaml_checker.py#L248-L264

While it's definitely not a proof that they're doing the same thing, it may be worth checking if there is any duplication across the two files.