CuVec v5.0.0 stable

• add shape property
• pybind11 minor updates
  • replace refs (&) => pointers (*)
  • implicit tuple => std::vector<size_t> conversion
• swig API updates
  • rename swvec => cuvec_swig
  • drop SwigCuVec_data
  • drop SwigCuVec alias in favour of NDCuVec
• cpython: (re)add backward-compatible header (#42 <- #40)
• docs: overhaul tabs & API
• build
  • default RelWithDebInfo
  • use pybind11_add_module <= python_add_library
• tests: speed up by using smaller sizes
• misc code tidy