扩展了LayerGroup，便于实现对其子图层的统一管理，如：
bindPopup binds a popup to all of the layers at once (likewise with bindTooltip)
Events are propagated to the FeatureGroup, so if the group has an event handler，因此FeatureGroup会处理来自子图层的鼠标事件和自定义事件、layeradd、layerremove
方法
- addLayer(layer)
- removeLayer(layer)
- setStyle(style)
- bringToFront()
- bringToBack()
- getBounds()
