Description

-Add folder selected in advanced NSRDB download dialog to list instead of rewriting list
-Folder is not added if already in library

To test:
-Open Detailed PV case
-Navigate to advanced NSRD downloads
-Go to choose folder dialog
-Select folder not in library
-Download file
-See file is added to folder list, list does not reset

Fixes #674

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

If you have added a new compute module in a SSC pull request related to this one, be sure to check the Process Requirements.

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings