NE3: Addition of third-party library: Yeppp!

[robbmcleod]

Yeppp! is a SIMD-based mathematics library from Georgia Tech that achieves similar performance to Intel’s VML, but Yeppp! comes under the permissive BSD license, the same as NE3.

TO DO: what is Yeppp!’s support for complex numbers? Can it handle strided arrays?

[robbmcleod]

Since Yeppp! is on Bitbucket it may be helpful to implement a CMake build for NumExpr3 that supports downloading it.

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