Transbigdata

Update the metro model, add functions:
- `transbigdata.metro_network`: create metro network
- `transbigdata.get_shortest_path`: Obtain the shortest path with given OD
- `transbigdata.get_k_shortest_paths`: Obtain the k shortest path with given OD
- `transbigdata.get_path_traveltime`: Obtain the travel time of the path

See example for detail usage: https://transbigdata.readthedocs.io/en/latest/gallery/Example%205-Modeling%20for%20subway%20network%20topology.html

update `plot_map`, now the basemap support input self-define mapbox style.

The `tbd.plot_map` function is added OpenStreetMap as the style 0, which do not need access token any more. imgsavepath and access_token are not neccessarily required now.

update `grid_params_optimize` method, this algorithm now is changed into PSO to speed up the optimizing process.

fix bug in `geohash_decode`

fix bug in `getadmin`