Greenland datasets: updated velocity data

[mkstratos]

MEaSUREs has updated data for ice sheet velocity and error components. (https://nsidc.org/data/NSIDC-0725/versions/1) It's available in GeoTIF format.

Make a method to handle this new input format and updated data, and ingest into the gridded data output.

[stephenprice]

The velocities linked to above are all very new. In discussions with Ian Joughin on this, he suggested that we most likely want to use these:

https://nsidc.org/data/nsidc-0670/versions/1

...which will be the most accurate, have the best coverage, and will have unwanted seasonal variability averaged out.

I've put a directory and a README file for these data (at NERSC) here:

/project/projectdirs/piscees/data/gis/joughin-InSAR-2020