NA Handling: Mechanistic model and more generally

[SOLV-Code]

Issue

In the Weighted Avg version of the mechanistic model (currently the default), NA handling is implemented as follows:

• drop incomplete rows (i.e. either the Abd or the Predictor has NA, or both)
• for the remaining records, sum the Abd and the Predictor, then calc the rate and do the rest

However, in most other functions, and more generally in the app, missing values are currently not allowed.

To Do

• check through the code and app to verify current set-up
• develop overarching approach (to ensure consistency as we add new model forms)
• document approach on the wiki