uncertainties are not easily calculated for components of composite models

[newville]

Description

See https://stackoverflow.com/questions/69683599/calculate-the-uncertainty-in-components-of-composite-model-in-lmfit/69716796#69716796

A request was made to be able to calculate the uncertainties in sub-models of composite models.

I think that is a reasonable request and probably not too hard to do. I'm not sure when I can get to this, but putting it here so it doesn't get forgotten. ;)