Add mock Donu compute error endpoint

[adamocarolli]

Description

Add mock Donu compute error endpoint. See Observable Notebook for understanding inputs/outputs: https://observablehq.com/d/f2195229ecf14e0c

Changes

• Add linear interpolation model
• Add L2 error model
• Add mocked Donu compute error function

Considerations

• Endpoint reflects the proposed functionality layed out by @liunelson

Testing

• The Observable notebook can be used for easier understanding. However you can also attach the compute error function to window with window.computeDonuModelSimulationError = computeDonuModelSimulationError and then test in browser.

Artifacts

• https://observablehq.com/d/f2195229ecf14e0c