auto-differentiation/xad: XAD v2.1.0

What's New xad-codegen JIT compilation support XAD now supports xad-codegen, a high-performance native code generation backend that compiles recorded computation graphs to optimized x86-64 machine code with AVX2 SIMD vectorization. New CMake option XAD_ENABLE_CODEGEN (default OFF) New CMake option XAD_CODEGEN_DIR to point to xad-codegen source directory xad::codegen::CodegenBackend and CodegenBackendAVX backend classes Updated LiborSwaptionPricerJIT sample xad-codegen is available under a separate commercial license. See XAD Enterprise for details.