treat Py_Finalize() more like MPI_Finalize()

this is done by
- not automatically calling Py_Finalize() when destructing a python interpreter
- adding wrapper functions so that the call to Py_Finalize() is hidden
  and skipped if Python support is no included.
- call the Python::finalize() wrapper in main.cpp (similar to the equivalent Kokkos function)
- add a wrapper of that call to the C library interface

src/library.h

@@ -95,6 +95,7 @@ void lammps_close(void *handle);
 void lammps_mpi_init();
 void lammps_mpi_finalize();
 void lammps_kokkos_finalize();
+void lammps_python_finalize();
 
 /* ----------------------------------------------------------------------
  * Library functions to process commands