ENH: introduce flushing

- this needs closing and re-opening the file because ADIOS2 does not implement flushing for BP5
https://github.com/ornladios/ADIOS2/issues/3060

src/foam/db/IOstreams/SliceStreams/FileSliceStream.C

@@ -23,3 +23,10 @@ void Foam::FileSliceStream::v_access()
     enginePtr_ = sliceFile_->createEngine(ioPtr_.get(), paths_.getPathName());
 }
 
+
+void Foam::FileSliceStream::v_flush()
+{
+    Foam::SliceStreamRepo* repo = Foam::SliceStreamRepo::instance();
+    repo->close();
+    v_access();
+}

src/foam/db/IOstreams/SliceStreams/FileSliceStream.H

@@ -19,6 +19,8 @@ class FileSliceStream
 
     virtual void v_access() final;
 
+    virtual void v_flush() final;
+
 public:
 
     explicit FileSliceStream(std::unique_ptr<StreamFeatures>&);

src/foam/db/IOstreams/SliceStreams/OFCstream.C

@@ -370,10 +370,7 @@ void Foam::OFCstreamBase::writeGlobalGeometricField()
 
     if (mode() == SYNC)
     {
-        // sliceStreamPtr->bufferSync(); // ToDoIO rename to "commit"
-                                // ToDoIO introduce "fileSync"
-        //auto repo = SliceStreamRepo::instance();
-        //repo->close();
+        sliceStreamPtr->flush();
     }
 
     if (Pstream::master())

src/foam/db/IOstreams/SliceStreams/SliceStream.C

@@ -261,4 +261,10 @@ void Foam::SliceStream::put
             );
 }
 
+
+void Foam::SliceStream::flush()
+{
+    v_flush();
+}
+
 // ************************************************************************* //

src/foam/db/IOstreams/SliceStreams/SliceStream.H

@@ -33,6 +33,8 @@ class SliceStream
 {
     virtual void v_access() = 0;
 
+    virtual void v_flush() = 0;
+
 protected:
 
     // Forward declaration of bridge to implementation
@@ -148,6 +150,8 @@ public:
 
     void bufferSync();
 
+    void flush();
+
 };
 
 }