Using the Eigen library in CUDA kernels

Eigen is a fantastic library, which since version 3.3 can be used inside CUDA kernels. This MWE shows the calculation of a sum of dot products using a std::vector<Eigen::Vector3d>. The USE_CUDA macro switches from the regular C++ (CPU) implementation to the CUDA implementation. CMake is used to configure the build and CORE_USE_CUDA can be used to switch between implementations.