MFIter::Finalize (AMReX-Codes#2983)

Add a Finalize function to MFIter.

The idea about this is, that we can call this already before destruction
in Python, where `for` loops do not create scope.

This function must be robust enough to be called again in the
constructor (or we need to add an extra bool to guard that it is not
called again in the destructor).

Co-authored-by: Weiqun Zhang <WeiqunZhang@lbl.gov>

Src/Base/AMReX_MFIter.H

@@ -180,6 +180,7 @@ protected:
     IndexType     typ;
 
     bool          dynamic;
+    bool          finalized = false;
 
     struct DeviceSync {
         DeviceSync () = default;
@@ -201,6 +202,7 @@ protected:
     static AMREX_EXPORT int allow_multiple_mfiters;
 
     void Initialize ();
+    void Finalize ();
 };
 
 //! Is it safe to have these two MultiFabs in the same MFiter?

Src/Base/AMReX_MFIter.cpp

@@ -209,6 +209,15 @@ MFIter::MFIter (const FabArrayBase& fabarray_, const MFItInfo& info)
 
 MFIter::~MFIter ()
 {
+    Finalize();
+}
+
+void
+MFIter::Finalize ()
+{
+    if (finalized) return;
+    finalized = true;
+
 #ifdef AMREX_USE_OMP
 #pragma omp master
 #endif