Update Northern Plains Case Study to use SMAP L4 Version 9 data

[arthur-e]

Version 9 includes significant improvements to the permanent water mask, which will help to improve results in this case study. The case study should also be updated to showcase the use of QA/QC flags in the data, for masking out water bodies. Currently, the Version 9 dataset does not include data from 2017; presumably, it will include these dates later as re-processing catches up.

[arthur-e]

Might also add some commentary on the use of afternoon versus morning acquisitions; from Colliander et al. (2022, IEEE JSTARS):

The accuracy requirement for L2SM products applies to the SM retrieved using the 6 A.M. (descending) SMAP overpasses because of the expected uniformity of the temperature across the soil-vegetation column [49], but the SM retrieved using the 6 P.M. overpasses is also validated (and actually has roughly equivalent accuracy performance to the AM overpasses). L4 SM is available and validated at 3-h intervals.