"layerLevel" attribute is used to represent the "place" for Layer in Model.

• In Sequential Model, the "layerLevel" will be the same as "layerIndex", and the layerLevel will be unique for all layers.
• In Functional Model, the "layerLevel" may be the same for several layers which has different "layerIndex".

layerLevel will use useful for dynamic model animation and relation lines in later development.