Code needed: don't re-download preexisting files

Add code to each of the GET_[type]_by_assetUid.py scripts to check if the raw data file for a particular month already exists; we don't need to download it again if so. DO NOT skip the download if the month is between the last update date and now--that file probably only has data from part of the month, and should be redownloaded for completeness.
