add decision utility funciton draft

[OriolAbril]

• Notebook follows style guide https://docs.pymc.io/en/latest/contributing/jupyter_style.html
• PR description contains a link to the relevant issue: taking a stab at Bayesian Decision Analysis (Posterior Predictive + Utility Function) #407
• Check the notebook is not excluded from any pre-commit check: https://github.com/pymc-devs/pymc-examples/blob/main/.pre-commit-config.yaml

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[drbenvincent]

• Looks great. I guess there will be some more explanation text added in the final draft?
• There's potentially scope for doing a simple model 1 with just one outcome variable as a really simple start, then bring in the model with 3 outcome variables to demonstrate the generality.
• Could be worth describing where the utility function comes from, or that this is up to what the person finds most important.

[OriolAbril]

I guess there will be some more explanation text added in the final draft?

Yes, this is only getting the code to work and sharing in case someone had suggestions on the model. I'll try to come back to this and add the explanation and finish t some point next week