Put date of model/forecast updates in header

[PhilMiller]

Users need to be able to tell when their previous reports need to be re-run to account for changes in the calculations. A date showing the last update to the modeling and forecasting code would support that.

[PhilMiller]

Notes regarding this need should be apparent in at least the following places:

• CONTRIBUTING.md
• Comments in headers of relevant computation files
• Pull request templates