New generalised masking strategy across SIC products

[JimCircadian]

The original paper guidance / approach isn't available and the masks were originally pulled from the product - in that case OSISAF. In order to be cross-compatible with differing ground truth products, I'd like to update the methodology to use, explicitly, the masks provided by NSIDC for this purpose. Comment / citation as follows:

Masking of regions to include/omit in dataset.

    NSIDC Land, Ocean, Coast, Ice, and Sea Ice Region Masks

    TODO: replace with NSIDC masks from:
     https://nsidc.org/data/user-resources/help-center/does-nsidc-have-tools-extract-and-geolocate-polar-stereographic-data#anchor-land-masks

    Citation
        Meier, Walter N., and J. Scott Stewart. (2023). NSIDC Land, Ocean, Coast, Ice, and Sea Ice Region Masks. NSIDC Special
        Report 25. Boulder CO, USA: National Snow and Ice Data Center.
        https://nsidc.org/sites/default/files/documents/technical-reference/nsidc-special-report-25.pdf.