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##### Add UI controls to switch layers on and off

Uses Leaflet's built-in http://leafletjs.com/reference.html#control-layers{layers control} feature to allow users to choose one of several base layers, and to choose any number of overlay layers to view.

##### Usage
addLayersControl(map, baseGroups = character(0), overlayGroups = character(0),
position = c("topright", "bottomright", "bottomleft", "topleft"),
options = layersControlOptions())layersControlOptions(collapsed = TRUE, autoZIndex = TRUE)removeLayersControl(map)
##### Arguments
map
the map to add the layers control to
baseGroups
character vector where each element is the name of a group. The user will be able to choose one base group (only) at a time. This is most commonly used for mostly-opaque tile layers.
overlayGroups
character vector where each element is the name of a group. The user can turn each overlay group on or off independently.
position
position of control: 'topleft', 'topright', 'bottomleft', or 'bottomright'
options
a list of additional options, intended to be provided by a call to layersControlOptions
collapsed
if TRUE (the default), the layers control will be rendered as an icon that expands when hovered over. Set to FALSE to have the layers control always appear in its expanded state.
autoZIndex
if TRUE, the control will automatically maintain the z-order of its various groups as overlays are switched on and off.
##### Aliases
leaflet() %>% addTiles(group = "OpenStreetMap") %>% addProviderTiles("Stamen.Toner",
"Toner by Stamen"), overlayGroups = c("Markers"))