Code to convert the native MNIST data format to PNG images.
Input arguments:
--image, -i: path to uncompressed mnist image file
--label, -l: path uncompressed mnist label file
--output, -o: path to a folder for saving the generated outputs
--randomise, -r: to indicate whether the output files' names should be randomised
--number, -n: indicates how many images form the input file should be processed
--help, -h: show usage and help message