Author: Makabongwe Nkabinde
operation with two images can be any of these {-a(add), -s(subtract), -l(mask)}
example: executable -l Lenna_standard.pgm Lenna_hat_mask.pgm maskOutput.pgm
threshold operates on one Image. threshold is an integer
example: executable -t shrek_rectangular.pgm 150 threshold.pgm
invert operation only works on 3 arguments.
example: executable -i Lenna_standard.pgm invert.pgm
The resulting image will be automatically saved once the operation is done.