Update for Mark's recent PdsFile changes

[rfrenchseti]

• Fixes #N/A
• Were any Django, import pipeline, table_schema, or dictionary files modified? Y
  • If YES:
  • Database used: opus3_test_210331
  • All Django tests pass: Y
  • FLAKE8 run on modified code: Y
• Were any JavaScript or CSS files modified? N
  • If YES:
  • JSHINT run on all affected files: Y/NA
  • Tested on Chrome, Firefox, Safari, Edge, and iOS: Y/NA (Remember to test all browser sizes)
  • Tested for race conditions (with delayed API calls if applicable): Y/NA

Description of changes:

Change is_volset() etc from functions to properties. Remove new stray '/' at the beginning of 'url' property.

Known problems:

None