Standardizing complex numbers in SYCL

Complex numbers are used in many high performance computing applications for scientific simulations. They were missing in the SYCL 2020 specification, resulting in fragmented and inconsistent implementations in the SYCL ecosystem. To address this, we devised an extension to the standard to provide a sycl::complex type together with operators and math functions, and developed a header only implementation of this extension with liberal open source licensing that can be used in any SYCL implementation.