Improve the efficiency

[cshjin]

• Build $\tau$ and $\eta$ within graph or in separate matrices?
• Clean up the memory consumption in DisjGraph updating
• Parallelize ACO (ACO_LS) to accelerate the convergence.
  • Buck update of pheromone given a set of ants
  • Local search in parallel (revise the direction based on solutions coming from a set of subgraphs)
  • Implement synchronization mechanisms to coordinate the actions of different colonies

[cshjin]

Refs to Parallel ACO:

• https://github.com/Tommliu/Parallel-Ant-Colony-System
• https://www.sciencedirect.com/science/article/pii/S0957417422013586
• https://www.sciencedirect.com/science/article/pii/S156849461100202X
• https://pure.bond.edu.au/ws/portalfiles/portal/30066807/AM_A_parallel_implementation_of_ant_colony_optimization.pdf