Fix neighbors cleanup message

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: vincent-noel

core/PhysiCell_cell.cpp

@@ -3416,7 +3416,10 @@ void Cell::remove_self_from_all_neighbors( void )
 			pC->state.neighbors.begin(),pC->state.neighbors.end(),this );
 			if ( SearchResult != pC->state.neighbors.end() )
 			{
-				std::cout << "Cell " << pC->ID << " still has cell " << this->ID << " as a neighbor!" << std::endl;
+				#pragma omp critical
+				{
+					std::cout << "Cell " << pC->ID << " still has cell " << this->ID << " as a neighbor!" << std::endl;
+				}
 				pC->state.neighbors.erase( SearchResult ); 
 			}