Add NLDAS-3 retrospective surface merge utility

[Kristen-M-Whitney]

Description

Add the NLDAS-3 retrospective surface merge utility under:

lis/utils/nldas-3_forcing/retrospective/surface

This PR adds:

• merge_files_for_date_range.f90, a Fortran utility that merges hourly LIS retrospective surface meteorology output files into one daily NetCDF file while preserving all 24 hourly timesteps.
• Updates to README.md documenting the merge utility, compilation and usage, and the retrospective surface workflow.
• Documentation describing the relationships among the dynamic_lapse_rates, retrospective/surface, retrospective/lwdown, and retrospective/merge workflows.

The merge utility packages the hourly surface meteorology outputs generated by the retrospective surface workflow into daily NetCDF files that serve as surface meteorology inputs to the downstream retrospective/merge workflow. The hourly LWdown_f output is used separately by the retrospective/lwdown workflow to generate the CERES-adjusted downward longwave radiation field used during final retrospective forcing assembly.

Resolves #1837

Testcase

Utility and documentation addition. No LIS build or regression tests were performed.

[Kristen-M-Whitney]

@dmocko @fadjimaina

FYI, this PR adds the retrospective surface hourly-to-daily merge utility (merge_files_for_date_range.f90) and updates the README to document how the outputs from this workflow fit into the broader NLDAS-3 retrospective forcing workflow.

Specifically, the README now describes:

• how the hourly LWdown_f output generated here by LIS is used by the lis/utils/nldas-3_forcing/retrospective/lwdown workflow before the final retrospective forcing assembly (@fadjimaina), and
• how the merged hourly-by-day surface meteorology files (one file per day, with all 24 hours) generated here serve as inputs to the downstream lis/utils/nldas-3_forcing/retrospective/merge workflow (@dmocko).

Hopefully this helps tie together the relationships among the three retrospective workflow directories as they're being populated.