Some minor fixes to the GNSS ZTD downloader

[jlmaurer]

Description

• Graceful failure if no stations in the input file are available from the source repo
• Handled the case where there are duplicate station names
• Fixed some path typos and type problems

Motivation and Context

Tried running GNSS downloader on a single example station and was getting failures.

How Has This Been Tested?

• unit tests pass locally

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Enhancement / cleanup (non-breaking change which makes things better)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have added an explanation of what your changes do and why you'd like us to include them.
• I have written new tests for your core changes, as applicable.
• I have successfully ran tests with your changes locally.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.