Interpolation function for validation

[ryardley]

Validation messages should be able to be calculated using an interpolation function.

This was a little technical and not critical to launch the validation feature so spinning it off to its own task here.

This idea first came from https://github.com/ryardley/pdsl/issues/22

This follows from work on the validation functionality PR here: https://github.com/ryardley/pdsl/pull/104