Add dynamic update capability for viewport based on data bounds

[bbriegel]

Allows dynamically updating the viewport based on changes in the data bounds of the layers, e.g.:

import gmaps

class MapContainer(object):
    def __init__(self):
        self._fig = gmaps.figure()
        self._marker_layer = gmaps.marker_layer(locations=[])
        self._fig.add_layer(self._marker_layer)
        display(self._fig)
        
    def update(self, coordinates_list):
        self._marker_layer.markers = [gmaps.Marker(location=coordinates) 
                                      for coordinates in coordinates_list]
        self._fig._map._calc_bounds({'new': self._fig._map.layers})
        
        
container = MapContainer()

london_coordinates = (51.507942, -0.127566)
berlin_coordinates = (52.518880, 13.408935)
new_york_coordinates = (40.708081, -74.005807)
washington_coordinates = (38.910675, -77.035177)

container.update([london_coordinates, berlin_coordinates])

container.update([new_york_coordinates, washington_coordinates])