Move custom GPU atomics into separate namespace. Add a script to print the include path.

• Move the custom atomicMin/Max/Mul functions to namespace pystencils::runtime::gpu, such that no name conflicts may ever arise with native CUDA/HIP functions
  • Other than CUDA, HIP defines atomicMin, atomicMax for floating-point types, so our definitions were raising duplicate-definition errors
• Use C++17 in cupy JIT compiler (for qualified namespace names)
• Make the pystencils.include module executable as a script to print the path to the pystencils header directory to stdout