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

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• 593132d9 - attribution

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requested review from @rmlarsen1

requested review from @cantonios

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• fb202a75 - make log2 constexpr

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resolved all threads

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• ca0077e0 - comments and some more loop types

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resolved all threads

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• e00f93f0 - various improvements

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• 64a39bc3 - dont bother threading head and tail of linear vectorized traversal

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• 753deb97 - fixes

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• 070c3767 - fixes

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• 74b0a459 - make threadcost a runtime variable

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• ff1f91ff - use gemm/v products by default

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