Fix tag for signed integers in vtu outputs

All integer values were marked as unsigned.
Was not noticed since output data were probably too small (<2^31)

src/output/VTKWriter.cpp

@@ -222,7 +222,7 @@ struct VTKWriter::VTKType
       if constexpr (std::is_unsigned_v<DataType>) {
         return "UInt" + stringify(sizeof(DataType) * 8);
       } else {
-        return "UInt" + stringify(sizeof(DataType) * 8);
+        return "Int" + stringify(sizeof(DataType) * 8);
       }
     } else if constexpr (std::is_floating_point_v<DataType>) {
       return "Float" + stringify(sizeof(DataType) * 8);