Add graph coloring algorithm

Amongst implemented in GraphX algorithm there is graph coloring missing. It should be added. At the beginning we can use simply greedy heuristic. In the next step we can consider implementing ie Boman graph coloring - https://www.researchgate.net/profile/Erik_Boman2/publication/221148539_A_Parallel_Distance-2_Graph_Coloring_Algorithm_for_Distributed_Memory_Computers/links/0912f50caa8822e82d000000.pdf