message: Make Python bindings build as a standaline project
  With this change it builds both as an option of the overall
  avogadrolibs project and as a standalone project.
diff: +cmake_minimum_required(VERSION 3.3 FATAL_ERROR)
  +
  +if (NOT TARGET AvogadroCore)
  + find_package(AvogadroLibs REQUIRED)
  +endif()
  find_package(PythonInterp 3 REQUIRED)
  find_package(PythonLibs 3 REQUIRED)
  set(PYBIND11_PYTHON_VERSION "3" CACHE STRING "")
  