Incorporate Threadpool in Eigen Core

Reference issue

What does this implement/fix?

Fixes #1044 (closed)

ThreadPool pool(2);
CoreThreadPoolDevice device(pool);
VectorXd a;
VectorXd b;
b.device(device) += a.cwiseAbs2().cwiseSqrt();
b.device(device).noalias() += a.cwiseAbs2().cwiseSqrt(); // useful for matrix products

// backwards compatibility
struct NonSupportedDevice{};
NonSupportedDevice otherDevice;
b.device(otherDevice) += a.cwiseAbs2().cwiseSqrt(); // still works (uses default eigen evaluators)

Additional information