Added CUDA error checking, algorithm runs in stages, and increased number of asteroids to 50.

CUDA error checking revealed that the kernel will fail if it runs for too long
(just by virtue of running for a long time, not due to any other error presumably),
so now the evolve() function runs the CUDA kernel in several stages, with a
configurable number of generations each time until it arrives at the desired
total number of generations.
Merge branch 'master' of github.com:quixoticdicker/GUDA