Proposed fix for jam values in table based files

[JoeGermuska]

I added a chunk of script at the bottom of the two table-based downloader files to strip out the specified "jam values" from data files, so that we don't inadvertently load spurious values next time we run these processes.

I've tested the code separate from actually loading the files into Postgres, but I figured it deserved more eyes, so submitting as a PR for @iandees review.