Combined image's uncertainty can use the wrong class

If I give `CCDData` objects with a `VarianceUncertainty` to the combiner, I get a `VarianceUncertainty` in the output object, but I guess this should be a `StdDevUncertainty` since this is what is computed (or the stddev values should be converted to the ouput object's uncertainty type).
Problem is that the values are copied directly to the output's uncertainty, though this one is copied from the input list and can be a different type.

https://github.com/astropy/ccdproc/blob/89db3547cbfb9ad953d28a3167e4773b86af1140/ccdproc/combiner.py#L428-L430

https://github.com/astropy/ccdproc/blob/89db3547cbfb9ad953d28a3167e4773b86af1140/ccdproc/combiner.py#L810

Start in ccdproc/combiner.py at the referenced lines 428-430 and 810, and trace how combined uncertainty values and their classes are selected. Confirm the behavior with CCDData inputs using VarianceUncertainty, then ensure the output uncertainty class and values agree with the computed representation.

Written by the indexing model from the issue text.