Fix GNSS data retrieval and downloading functions

[sssangha]

Updated the get_stats_by_llh function to prioritize IGS20 coordinates over legacy data.

Enhanced the download_UNR function to include a fallback to the legacy archive when downloading tropospheric delays.

Context: There appears to be ongoing migration of data in the UNR archive such that data from much of the eastern hemisphere is disrupted after 2023. This PR is intended to circumvent this.

Screenshots (if appropriate):

Type of change

• Bug fix (non-breaking change which fixes an issue)
• 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.