scaling up NEX assessment part 2, addding RMSE visualization with raster for 75% overlap run

scaling up assessemnt part 1 testing 75% overlap in North Africa

keeping tracked of stations removed and debugging

cleaning up and testing

spatial subsampling function and random sampling

initial commit for spatial subsampling code for NEX tile with many stations

run6 assessment NEX part2: generating raster map of accuracy and number of stations

run6 assessment NEX part2: adding new functions to generate raster map of accuracy overall and daily

run6 assessment NEX part2: add new plots of assessment of first global run

run6 assessment NEX part1: first global run with specific k