values should be object of unit and uncertainty

an easy extension to also incorporate uncertainty is to use the unit object and add a `std` key. Unfortunately, also this is not really general as it assumes a normal distribution.

I suggest

```
SI (number): value in SI units
unit (string): default units
std (number): standard deviation in SI units
```

Start by locating the existing definition of the values field in the data_schema repository and review how units are currently represented. Compare the proposed SI, unit, and std structure with existing schema usage, then determine and document the accepted uncertainty model before considering implementation.

Written by the indexing model from the issue text.