RF: since argument could be not just a list but a package id etc, renamed into items

[yarikoptic]

To me --help now is a bit more informative/less misleading

$> downloadcmd --help                                                                                        
Running NDATools Version 0.2.3
Opening log: /home/yoh/NDAValidationResults/debug_log_20201113T103250.txt
usage: downloadcmd <items>

This application allows you to download from NDA. Depending on use of -dp, -t, or -ds flags, expected <items>
would differ. You could specify a list of aws S3 paths and will download the files to your drive in your home
folder. Alternatively, you may enter a packageID, an NDA data structure file or a text file with s3 links, and
the client will download all files from the S3 links listed. Please note, the maximum transfer limit of data is
5TB at one time.

positional arguments:
  <items>               Will download all S3 files to your local drive
...