update file downloads to create metadata provenance file

[ecoon]

There is significant concern about the provenance of some of the data files that are being downloaded. Not all of them are formally versioned, in the sense that, e.g., NHDPlus data is frequently updated/cleaned/changed without a version number changing.

WW provides some provenance here -- we download the file once and don't change it, so you can know exactly that you're getting repeatable data. But to make it traceable for someone else, they would have to get the same dataset. We can't perfectly fix this, but it would be useful to at least log, with every file download, some basic metadata about the file being downloaded (e.g. url, request, timestamp, etc).