1070 allow selection of download file output format

[juzen2003]

• Fixes #1070
• Were any Django, import pipeline, table_schema, or dictionary files modified? N
  • If YES:
  • Database used: re-import with the latest pds-webtools
  • All Django tests pass: Y
  • FLAKE8 run on modified code: Y
• Were any JavaScript or CSS files modified? Y
  • If YES:
  • JSHINT run on all affected files: Y (cart.js)
  • Tested on Chrome, Firefox, Safari, Edge, and iOS: Y (Remember to test all browser sizes)
  • Tested for race conditions (with delayed API calls if applicable): NA

Description of changes:

• Add a select input (default: zip) in cart download pane for users to select download file format (zip/tar/tgz).
• Add the same zip file tests for the other two formats.

Known problems: