图层模块，包含各种类型的图层管理功能
Classes
- BaseLayer
- ClusterLayer
- DivLayer
- FeatureCollection
- FeatureStyleParser
- GeoJsonFeatureCollection
- GeoJsonLayer
- GltfLayer
- GmlLayer
- GraphicLayer
- GroupLayer
- GSMMLayer
- I3STilesLayer
- ImageryLayer
- InstanceLayer
- InstanceLayerModel
- KmlLayer
- LayerManager
- S3MTilesLayer
- TerrainLayer
- Tiles3DLayer
- TransformableGroupLayer
- USDLayer
- VectorTileLayer