Apply noise or remove it

[kwinkunks]

If our datasets are so clean that additive noise might be useful, then let's keep it and add it in to all of the examples. (It's not being used in the first sand/shale example.)

Or, if our datasets are already noisy, maybe we should just remove it, or at least rescale it so that the max is not so noisy.

[kwinkunks]

I think it is a nice feature, at least of the synthetic datasets — so we should make it do something for the real datasets.

Calling it a bug b/c I think that's how a user would see the current behaviour.

Alternative is to disable the slider (grey it out) for real datasets.

[kwinkunks]

Disabled for custom datasets by 23423d1

It probably looks even more like a bug now though 😬

No change yet for real datasets.