Combined image's uncertainty can use the wrong class

[saimn]

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