add new snow compaction option & revise README doc

[cenlinhe]

This PR is to add new snow compaction option in HRLDAS part to work with the recent updates in Noah-MP source code (NCAR/noahmp#174) as well as revise some incorrect information in README documents.
This PR was originally submitted as #219, but revised in this new PR here.
his new physics option can be used by defining SNOW_COMPACTION_OPTION=2 in the namelist file.

Reference for the new snow compaction scheme: Abolafia-Rosenzweig, R., He, C., Chen, F., Barlage, M., 2024. Evaluating and Enhancing Snow Compaction Process in the Noah-MP Land Surface Model. J Adv Model Earth Syst 16, e2023MS003869. https://doi.org.10.1029/2023MS003869

This new snow compaction scheme from Abolafia-Rosenzweig et al 2024 updates the original Noah-MP snow compaction scheme based on Anderson (1976). This scheme has been added to Noah-MP as a new physics option: "snow_compaction_option".
snow_compaction_option = 1 (original Noah-MP snow compaction scheme based on Anderson 1976);
snow_compaction_option = 2 (updated Noah-MP snow compaction scheme based on Abolafia-Rosenzweig et al 2024);
The default snow compaction scheme is currently set to the updated scheme (snow_compaction_option = 2).

Effects of this change: the new scheme generally enhances snow compaction rate, which leads to a better simulation for snow depth and snow density comparing with the SNOTEL observations over the western US.

A test run over CONUS using NLDAS-2 forcing on NCAR's HPC (Derecho) was successful.

[tslin2]

Should these namelists in the examples directory also be updated?
https://github.com/NCAR/hrldas/tree/master/hrldas/run/examples

[cenlinhe]

Should these namelists in the examples directory also be updated? https://github.com/NCAR/hrldas/tree/master/hrldas/run/examples

Yes, you are right. I will update those files as well.

[cenlinhe]

I updated the example namelists with the new snow compaction option

[RAbolafiaRosenzweig]

Snow compaction parameters should also be added to NoahmpTable.TBL in the hrldas run directory.

[cenlinhe]

Snow compaction parameters should also be added to NoahmpTable.TBL in the hrldas run directory.

the parameters have already been added to NoahmpTable.TBL, and NoahmpTable.TBL file is in the noahmp GitHub (not HRLDAS GitHub): https://github.com/NCAR/noahmp/tree/develop/parameters

[cenlinhe]

@RAbolafiaRosenzweig NoahmpTable.TBL in the hrldas run directory has a soft link to the NoahmpTable.TBL file in noahmp/parameters/ in noahmp github

[RAbolafiaRosenzweig]

Code updates have been made to allow the new snow compaction physics option from Abolafia-Rosenzweig et al. (2024).