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/PYTHON/python_impl.cpp

@@ -63,10 +63,6 @@ PythonImpl::PythonImpl(LAMMPS *lmp) : Pointers(lmp)
 #endif
 #endif
 
-  // one-time initialization of Python interpreter
-  // pyMain stores pointer to main module
-  external_interpreter = Py_IsInitialized();
-
 #ifdef MLIAP_PYTHON
   // Inform python intialization scheme of the mliappy module.
   // This -must- happen before python is initialized.
@@ -108,12 +104,6 @@ PythonImpl::~PythonImpl()
     }
   }
 
-  // shutdown Python interpreter
-  if (!external_interpreter) {
-    PyGILState_Ensure();
-    Py_Finalize();
-  }
-
   memory->sfree(pfuncs);
 }
 
@@ -545,3 +535,10 @@ bool PythonImpl::has_minimum_version(int major, int minor)
 {
     return (PY_MAJOR_VERSION == major && PY_MINOR_VERSION >= minor) || (PY_MAJOR_VERSION > major);
 }
+
+/* ------------------------------------------------------------------ */
+
+void PythonImpl::finalize()
+{
+    if (Py_IsInitialized()) Py_Finalize();
+}