Update CMake configuration for CUDA

The existing CMake code uses a custom FindCUDA module. This module was originally added when we still used CMake 2.x. FindCUDA existed since CMake 3.0, but is deprecated by now. CMake 3.17 introduces a new method: FindCUDAToolkit. To make IDG more future-proof, our custom FindCUDA module have been removed, and the CMake configuration has been updated to work with FindCUDAToolkit. In case an old CMake version is used, the built-in FindCUDA module is used and targets are created to match the ones that FindCUDAToolkit provides. Since we have CMake 3.7 as the minimum required version, we can be sure that the FindCUDA module is always available for this fallback.