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/lmppython.cpp

@@ -126,3 +126,12 @@ bool Python::has_minimum_version(int major, int minor)
   init();
   return impl->has_minimum_version(major, minor);
 }
+
+/* ------------------------------------------------------------------ */
+
+void Python::finalize()
+{
+#if defined(LMP_PYTHON)
+  PythonImpl::finalize();
+#endif
+}