Replace land mask with land fraction in the C++ IMS preprocessing code

[jiaruidong2017]

The GFS system, including the global workflow, GDASApp, and GSI, consistently uses land fraction rather than a binary land mask for land DA. For consistency with the rest of the system, we propose replacing the land mask with land fraction in the C++ IMS preprocessing code.

Related issue: #41

• Ensure consistent land/sea mask with land DA NOAA-EMC/GFS#8

Related PRs

• regrid_sfc - separate non-land and water masks ufs-community/UFS_UTILS#1084 - Merged
• Soil mask NOAA-EMC/GSI#906 - Merged
• Replace slmsk with fraction_of_land NOAA-EMC/jcb-gdas#146 - Merged
• Use land mask from land fraction instead of from restarts NOAA-EMC/land-apply_jedi_incr#3 - Merged
• IMS preprocessing (This issue)

[jiaruidong2017]

PR: NOAA-EMC/GDASApp#2079