resolves broken builds (gh-198); but this is a patch. There is an underlying issue with query.covariates that causes it to return an empty dataframe (line 430 of query.trait.data.R); there is also a hack to put canopy layer covariates in a separate dataframe because otherwise filter.sunleaf.traits does not work
Merge pull request #199 from PecanProject/GH-198-fix-query-traits

resolves broken builds (gh-198); but this is a patch.