fixed signed / unsigned issue in LevelGridBuilder

src/gpu/GridGenerator/grid/GridBuilder/LevelGridBuilder.cpp

@@ -216,13 +216,13 @@ void LevelGridBuilder::getSlipValues(real* normalX, real* normalY, real* normalZ
     int allIndicesCounter = 0;
     for (auto boundaryCondition : boundaryConditions[level]->slipBoundaryConditions)
     {
-        for (int i = 0; i < boundaryCondition->indices.size(); i++)
+        for (uint index = 0; index < boundaryCondition->indices.size(); index++)
         {
-            indices[allIndicesCounter] = grids[level]->getSparseIndex(boundaryCondition->indices[i]) + 1;
+            indices[allIndicesCounter] = grids[level]->getSparseIndex(boundaryCondition->indices[index]) + 1;
 
-            normalX[allIndicesCounter] = boundaryCondition->getNormalx(i);
-            normalY[allIndicesCounter] = boundaryCondition->getNormaly(i);
-            normalZ[allIndicesCounter] = boundaryCondition->getNormalz(i);
+            normalX[allIndicesCounter] = boundaryCondition->getNormalx(index);
+            normalY[allIndicesCounter] = boundaryCondition->getNormaly(index);
+            normalZ[allIndicesCounter] = boundaryCondition->getNormalz(index);
             allIndicesCounter++;
         }
     }