EAMxx: better configuration of fill_threshold when it comes to fillval treatment

[mahf708]

See https://github.com/E3SM-Project/E3SM/issues/7326 for context as an example of the issue.

[bartgol]

I think we should just set the default to 0 and move on. This is a power user feature, so the default should not assume any arbitrary threshold. Using 0 means "as long as 1+ snapshot has valid values, we can compute an average", which is imho the best base strategy.