label daily preds unc as SE, not SD

GPP.daily.sd, etc. are standard errors, aren't they? not sure why i've used SD all this time, except maybe that they come from the SDs of the MCMC chains

Start by locating the code and documentation that define or display GPP.daily.sd and related daily prediction uncertainty fields. Confirm whether these values are standard errors rather than standard deviations, then update the labels to SE and verify the terminology consistently across the affected outputs.

Written by the indexing model from the issue text.