message: CMake is back to C++17
diff: @@ -2,7 +2,7 @@ cmake_minimum_required (VERSION 3.5)
  project (DataFrame VERSION 1.0.0)
  set(CMAKE_EXPORT_COMPILE_COMMANDS ON) # Produce cmpile_commands.json
  -set(CMAKE_CXX_STANDARD 20)
  +set(CMAKE_CXX_STANDARD 17)
  set(CMAKE_CXX_STANDARD_REQUIRED ON)
  set(CMAKE_POSITION_INDEPENDENT_CODE ON)
  