[FEATURE] Add per-thread queuing for benchmarking

[ell0ry]

Some planning setups require that a planner be made for each thread and maintained. For example, with my current DART + Robowflex setup, I need to interact with the underlying DART scene for different queries.

This limits my ability to parallelize benchmarking as I need to enforce that each worker thread only interacts with its respective planner. I've got a workaround for the time being, but it would be nice to have the ability to queue runs in the Experiment API and assign them to a particular worker thread.

Detailed Description

Allow requests to be queued in an Experiment and assigned to a particular thread.

Possible Implementation

The current implementation accents a number of worker threads in the Experiment::benchmark (std::size_t n_threads=1) method. Instead, this param would need to be accepted in the constructor, and the various Experiment::addQuery methods be changed to accept a thread index to queue the job to.

Under the hood, the queued jobs will need to be indexed by their assigned thread. Currently, the Experiment::benchmark method has each thread grab a mutex lock, pop a job to run from the queue, and unlock. If the jobs are already indexed by their worker thread, each thread can pop a job off its queue without worrying about synchronization.

This would leave the burden of dividing the jobs up by thread to the user. However, I think this might be useful in fringe situations like what I'm dealing with.

[ell0ry]

Because this change requires that data structures and function signatures inside the Experiment class be changed, I'll need to copy it instead of extending it. For now, I'm going to work on a pull request that implements a cloned and updated ThreadedExperiment class in the same source file.