Add Pine Island Glacier dataset

[weiji14]

Add in high resolution Pine Island Glacier dataset from British Antarctic Survey's Ice Sheet Stability Programme (iSTAR).

• Get iSTAR DELORES DEMs from Rob Bingham
• Process 2013-2014 iSTAR raw bed pick data to DEM (be0b77c)
• Process 2010 raw bed pick data to DEM (be0b77c)
• Tile many 250m resolution DEMs (38143f3)
• Retrain FSRCNN on just Pine Island Glacier dataset (325d272)
• Use iSTARt6 and 2007 DEMs as independent test set for model verification (TODO Migrate to new issue).

References:

• Bingham et al. 2018 paper
• https://istar-gis-features.data.bas.ac.uk/glacial/delores_dems/
• WAIS workshop presentation