fix LinAlgError in HSGP-Basic.ipynb, drop find_constrained_prior

[Armatron44]

Fixes #800. In addition, I spotted that the to be depreciated find_constrained_prior was still being used in the linearized HSGP model example, so I switched for the preliz max_ent approach.

I've specified the number of chains to run to match old version of notebook (my old pc will only run 2 by default). I also suppressed some user warnings from matplotlib by dropping calls to plt.tight_layout and plt.subplots_adjust.

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📚 Documentation preview 📚: https://pymc-examples--801.org.readthedocs.build/en/801/

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[AlexAndorra]

Looks good; good catch @Armatron44 !
I'll give @bwengals a few hours before merging, in case he wants to add something, but the stabilization of the covariance matrix looks good to me.