added automatic fill missing data functionality in ensureCorrectDimensions and updated format_nc_files so that landfrac and SNWHLND are automatically filled in with zeros when NaNs are present (such as over oceans). Also updated SNWHLND so that numbers are divded by an assumed snow density of 500kg/m3 and updated O3 so that numbers are divided by 10^9 since units must be converted from ppb to molar fraction.