Add layer control for leaflet map

Add layer control for leaflet map

add_control(leaf, layer_vec, tiles_info = TILES_INFO)

Arguments

leaf	leaflet map widget object.
layer_vec	character vector of user data layers. For example animal names, model names etc.
tiles_info	A list holding tiles information, default to be ctmmweb:::TILES_INFO. To customize it, use ctmmweb:::TILES_INFO as a template.