Implement str-type uncertainty for the BlueiceExtendedModel

[hammannr]

Initially, I thought we could simply get away with evaluating the str if it starts with numpy. or scipy. but for the variation of the ancillary measurements, we'd need to know which of the parameters corresponds to the measured value.
One option to solve this would be to enable a wildcard $MEASUREMENT$ in the str (e.g. uncertainty: "scipy.stats.uniform(loc=-1+$MEASUREMENT$, scale=2" or "scipy.stats.skewnorm(a=4, loc=$MEASUREMENT$, scale=2"). But also not specifying it might be ok, e.g. if there are physical bounds and the measurement doesn't affect the constraints, maybe in this case we just want to raise a warning at the beginning or something like this.

But I don't know if this is the most user-friendly approach so if anyone has a better suggestion just let me know! 😊