Pr239 fixes

1. Fix cross-tab handling
2. Fix doubled URL in precip table

Getting this one working was a little tricky due to both our poor documentation of the cross-tab functionality and to a bug https://github.com/weecology/retriever/issues/241 that your PR lead me to discover. So, instead of walking you through a bunch of confusion introduced by us I thought I'd just fix this and send the changes your way. If you merge this it will update the PR and then I'll go ahead and merge it in.