add nexrad source functionality

[iskandari]

Adds support for retrieving NEXRAD-based VPTS data in getRad with:

• nexrad as a new source in get_vpts() and get_vpts_coverage()
• get_vpts_nexrad() and get_vpts_coverage_nexrad() helpers
• integration with the public BirdCast S3 VPTS archive
• radar/date coverage validation
• caching support for NEXRAD coverage metadata
• comprehensive tests for coverage retrieval, validation, and VPTS downloads
• updated documentation and examples for NEXRAD usage
• incidentally fixes the Estonia PVOL test by making the download failure-tolerant for intermittent 429/500 API responses.

[codecov]

Codecov Report

❌ Patch coverage is 98.83721% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.75%. Comparing base (f4bc216) to head (82f1494).

Files with missing lines	Patch %	Lines
R/zzz.R	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #177      +/-   ##
==========================================
+ Coverage   91.66%   91.75%   +0.08%     
==========================================
  Files          25       27       +2     
  Lines        1896     1976      +80     
==========================================
+ Hits         1738     1813      +75     
- Misses        158      163       +5     

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[bart1]

Hi @iskandari Great to see all activity here! I hope to have some time later to actually also test this! For my reference this is a different dataset from #175 right? Or is it the same data?

[iskandari]

@bart1 it is a different dataset that we recently made public on S3

[adokter]

This is the vol2bird/bioRad dataset

[adokter]

@bart1 @peterdesmet please coordinate with me on the exact timing of merging/releasing this - good to have this available before the IRAC workshop but not fully unannounced.

[bart1]

@bart1 it is a different dataset that we recently made public on S3

Thanks for the update, good to understand the differences!