function agent(lat_lng, options, agentmap) {
  return new Agent(lat_lng, options, agentmap);
}
Returns an agent object.

@param {LatLng} lat_lng - A pair of coordinates to locate the agent at.
@param {Object} options - An array of options for the agent, namely its layer.
@param {Agentmap} agentmap - The agentmap instance in which the agent exists.