Check file exists before download

[wveit]

Addresses #17 - Where a failed download during a subscriber run would cause all the successfully downloaded files to be re-downloaded during the next run.

Changes:

• Before downloading a file, subscriber first checks if the file already exists in the output_directory. If it does exist, calculate a checksum for the file and check it against the checksum that was in the granule search results. If the checksum matches, then skip downloading that file.
• This new behavior can be overridden with a new --force/-f option, which will cause all files that shows up in the CMR search results to be downloaded

Effectively, this change does something like this:

if (   the_file_exists() and 
       the_file_checksum_is_current_with_cmr() and 
       force_option_not_used()  ):
     skip_the_download()
do_the_download()

I should mention, this PR doesn't really do anything special that causes the subscriber "resume" or "retry" if a failure occurs. It just prevents re-downloading files on the next run.

[frankinspace]

Looks good to me. Kudos on the documentation and tests as well, looks great.