Corrected namespace in 01_cross_with_circle
01_cross_with_circle: added empty -main stub function
gen_art/02-09: added -main stub function
gen_art/10-20: added -main stub function
gen_art/21-26: added -main stub function
27_noise_perspective: corrected namespace and added -main stub function
gen_art/28-31: added -main stub function