Implement MODIS snow cover fraction assimilation

[gmao-rreichle]

@jpark50 is implementing the MODIS snow cover fraction assimilation of Toure et al. (2018), doi:10.3390/rs10020316 in GEOSldas.

Ally's original implementation dates back to ~2012 and was never merged into the main development branch.
The original implementation was added on top of a very outdated version of LDASsa (reichle-LDASsa_m2-09, predating the earliest version mentioned in the GEOSldas CHANGELOG.

A brief analysis of the differences and the location of the LDASsa source code is here:
readme_AllyToure_LDASsa_MODIS_SCF_assimilation_20210708.txt

cc: @lcandre2 @weiyuan-jiang @biljanaorescanin