Added showcase notebook for huber loss

[msaca-okse]

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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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[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).