Layer: FeatureGroup

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()