Add CERES SWdown metforcing reader for NLDAS-3 #1669
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This commit adds a metforcing reader for the CERES SWdown product over the NLDAS-3 domain. The product is binary little_endian real*4 hourly at 4-km over the domain.
This metforcing reader will read in the product and enable interpolation and slope-aspect corrections to the running grid. Primarily, it will be run on the 1-km NLDAS-3 grid.
Note that this metforcing reader should NOT be merged into the main branch at this time.
Resolves: #1668