Add support for std::thread to differentiate concurrent programs

[vaithak]

When differentiating a function which uses multithreading as the concurrency mechanism, clad should be able to generate the derivative code which exploits a similar concurrent structure.

Good examples to test this could be map-reduce style implementations of algorithms.

[vgvassilev]

@pmozil, that's an issue in the context of your GSoC project. Can you put it in your work plan?

[pmozil]

Yeah, np, will add the issue to the plan rn