Low-level planner parallelization

[zijinoier]

Description

Although there is no good idea to parallelize the high-level body conflict tree for the time being, it is feasible to parallelize the low-level planner. It will not only speed up at the root node when we plan for all the agents with no constraints. Also, we can now popped up more than one node a time to extend the search tree, since the single planner is parallelized. This will certainly increase the search speed of our algorithm. The impact of this modification on the algorithm's completeness remains to be examined.

Proposal

To be decided.