Slightly more coverage and regenerate grid plots! Explicitly setting the mask flag '-M' in GMT `surface` to 2c (see https://gmt.soest.hawaii.edu/doc/latest/surface.html#m) meaning a 5x5 cell neighbourhood mask. This has an effect similar to (but not exactly like) the window_size option in pdal we used before (see https://pdal.io/stages/writers.gdal.html#window-size). Plots re-rendered to show new 2007tx.nc plus updated areas for 201x_Antarctica_Basler.nc (now includes 2017_Antarctica_Basler.csv after 7a48d6a).