MFIter: Finalize() on StopIteration

Since `for` loops create no scope in Python, we need to trigger
finalize logic, including stream syncs, before the destructor of
`MultiFab` iterators are called.

src/Base/MultiFab.cpp

@@ -95,6 +95,7 @@ void init_MultiFab(py::module &m) {
                 if( !mfi.isValid() )
                 {
                     first_or_done = true;
+                    mfi.Finalize();
                     throw py::stop_iteration();
                 }
                 return mfi;