bug correction for particle in/out mask access
modified:   PGlml.cpp: check if particle is inside image during mask test, i.e. in/out mask.
cleaner code using containsXYZV CImg library function.
modified:   PGlml.cpp: clean code with CImg librry function for checkin taht a point is inside image bound.
add mask for wall (i.e. no particles in walls)
modified:   PGlml.cpp: add mask example usage.
new file:   mask.png: mask of the same size as image (value: 0 = wall).