Add underflow discussion to HSGP-Basic

[Armatron44]

Hi all,

Added a section to address underflow in HSGPs as discussed here pymc-devs/pymc#7881. There's also a typo fix in the markdown cell that discusses m and c for matern52.

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📚 Documentation preview 📚: https://pymc-examples--803.org.readthedocs.build/en/803/

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[AlexAndorra]

Thanks @Armatron44 ! I'll review ASAP

[AlexAndorra]

This is great @Armatron44 , thanks a lot! I left a few comments in ReviewNB, and as an overall ask: can we make this into an admonition block (see the style guide?
With a title like "Click if you get this XX issue". This is a very niche and technical issue, and I'm afraid having it displayed by default in the notebook will break the flow, especially as it's a beginner NB. Does this make sense to you?

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AlexAndorra commented on 2025-08-15T20:57:33Z
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Line #1.    m52_L = m52_c * 50  # c * s

Maybe just comment quickly on why s = 50

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AlexAndorra commented on 2025-08-15T20:57:34Z
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Line #14.    # check non have underflowed to zero.

Typo: non --> none

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AlexAndorra commented on 2025-08-15T20:57:34Z
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Line #18.    )  # this will not pass assertion.

Maybe add an assertion error message? So that it displays better when failing

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AlexAndorra commented on 2025-08-15T20:57:35Z
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Typo: "as you need a m large to cover small lengthscales" --> "as you need an m large enough to cover small lengthscales"

[Armatron44]

Thanks @AlexAndorra for the comments and suggestions. I agree it hinders the flow of an introductory notebook so will try the admonition block as suggested. I've not much time this week so may have to leave this until next week to address.