Choose a more descriptive parameter name in place of `i` in `UiLayer._append_methods_list()`

[yousefmoazzam]

Typically single letter variable/function parameter names are discouraged, a more descriptive name here could likely be given: https://github.com/DiamondLightSource/httomo/blob/ae5610a83a691436e0e7deff3a71c2af3f89568e/httomo/ui_layer.py#L76-L87