import script doesn't check for existence before re-downloading

[jpiesing]

Split out from the Zach Cava comments in #38 .

o Rerunning import the script doesn't appear to check for existence before re-downloading, might be a good idea again given the size

[jpiesing]

February 20th meeting
Don't know what is in the .zip files so need to download them to check if the contents have changed.
Need another mechanism to check this.
Could perhaps create a hash of the filename.
Download has just been optimised today.