Add a general-purpose fork-join class

[elvout]

Additions

• concurrent directory and namespace for concurrency-related constructs.
• General-purpose ForkJoin class that accepts abstracts synchronization for the user.
  • Accepts derived classes of ForkJoinTask (essentially functions with arguments stored as data members).
  • Documentation is pretty basic right now, but I think the API is fairly straightforward.

Notes

In the future, we probably want to use thread pools since creating a separate ForkJoin instance and spawning multiple threads for each task is resource-intensive and may cause fairness issues.