Refs #10370 Added reserve back to loadAndAddfrom in MDBox.

This means the reserve will still be performed if required, but not if memory was already reserved.

Code/Mantid/Framework/MDEvents/src/MDBox.cpp

@@ -934,6 +934,9 @@ namespace MDEvents
        std::vector<coord_t> TableData;
        FileSaver->loadBlock(TableData,filePosition,nEvents);
 
+       // convert loaded events to data;
+       size_t nCurrentEvents = data.size();
+       this->data.reserve(nCurrentEvents+nEvents);  // if space already reserved, e.g. from MergeMDFiles, this does nothing
        // convert data to events appending new events to existing
        MDE::dataToEvents(TableData,data,false);