Revisit special handling of `seedling_mortality` in run.meta.analysis

[infotroph]

Current lines 103-104 of run.meta.analysis.R:

prior_variances[names(trait_average), ] <- 0.001 * trait_average ^ 2
  prior_variances["seedling_mortality", 1] <- 1

@infotroph in a comment on #3278:

Would it be possible to add a few comments here explaining (1) where the constants 0.001 and 1 come from, and (2) why seedling mortality gets special handling and whether that's a hack or a principled design choice?

On scrolling down further I see this is pre-existing code moved up from run.meta.analysis.pft, so maybe these explanations are lost to time...

@ashiklom replied:

Yeah, this is pre-existing code. The oldest git blame gets me is here:

https://github.com/PecanProject/pecan/blame/bed27a2b37619c2297c3c66becf97b8c16233494/modules/meta.analysis/R/run.meta.analysis.R

...but I'm almost certain this logic predates that, which looks more like refactoring than development.

The more correct thing to do here is probably (1) remove the seedling_mortality hard-coded value altogether and replace it with a global default prior of some kind in the database; (2) pass the 0.001 as an (optional) argument called something like default_prior_variance_fraction with 0.001 as the default value.