Enhance ERA5 Download Function: Direct NetCDF, Flexible Parameters, and Improved Docs

[divine7022]

Description

This PR refactors the download.ERA5_cds function to:

• Download ERA5 data directly in netcdf format (removes grib_to_netcdf conversion dependency).
• Parameters are now passed in the correct vector format to ensure proper API requests.
• Added support for flexible selection of product type, dataset, and time steps (no longer hardcoded).
• Fixed the extent argument to ensure correct spatial subsetting as required by the API.
• Provided clearer, beginner-friendly documentation and example usage.

Motivation and Context

Review Time Estimate

• [ ] Immediately
• [ ] Within one week
• [ ] When possible

Types of changes

• [ ] 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 change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] My name is in the list of CITATION.cff
• [ ] I agree that PEcAn Project may distribute my contribution under any or all of
  • the same license as the existing code,
  • and/or the BSD 3-clause license.
• [ ] I have updated the CHANGELOG.md.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.