Create model selection SOP

[dfsnow]

We should write a Standard Operating Procedure (SOP) codifying how and why we select final valuation model runs. This is mostly about formalizing and documenting the best practices and making sure that we're implementing them internally.

• [ ] Collect examples of similar SOPs from other departments/companies
• [ ] Collect best practices re: model selection, see Tidymodels docs, Max Kuhn, other predictive modeling resources
• [ ] Seek feedback from Valuations on any proposed changes
• [ ] Publish the SOP to the wiki, with link from README

[dfsnow]

We should also reconsider using our current method for bootstrapped CI calculations. We rarely use the CIs now and I'm not confident that the original implementation works as intended. See https://stats.stackexchange.com/questions/433866/bootstrapped-confidence-intervals-for-performance-metrics-of-predictive-models.