Added showcase notebook for huber loss

[msaca-okse]

Describe your contribution

I have made a small user showcase notebook, that shows the effect of using the Huber loss function over a least squares data fidelity.

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• I have performed a self-review of my code
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[paskino]

The original data of the steel wire is https://github.com/DiamondLightSource/Savu/raw/refs/heads/master/test_data/data/24737_fd.nxs

This is a NeXuS file from Diamond Light Source and it's not normalised and it has one outlier in the top detector row which we removed in our test data (and transformed to absorption).

[lauramurgatroyd]

Hi @msaca-okse thank you for your contribution!
The notebook looks great. I noticed a couple of very minor typos - it won't let me suggest these changes so I will try to edit the file myself and push the change.
I will also be looking at your Huber loss PR to CIL. We will aim to get the Huber Loss function into CIL, before merging this showcase.