Fix quadrature compilation issues with gcc 6.3.1-1
Definition of static constexpr double in TriangleQuadrature structs were behaving differently between different versions of gcc. The latest gcc 6.3.1-1 didn't like the fact that the ODR was being violated, whereas some previous versions didn't care. The definitions need to be visible in the Physics headers, so the quadrature rules were moved to a separate library and the definitions were moved to a cpp file.
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