v0.8.1

v0.8.1 mainly optimizes class ColumnVec, setup_agents() in class Network, and column generation module (i.e., colgen.py) to boost the performance in setup_agents() and path-base UE.

1. replace the data container in ColumnVec to store columns from dict to list and change implementations involving loops through stored columns in colgen.py and utils.py;
2. loop through column pool directly in class Network setup_agents();
3. remove check on cv.get_od_volume() <= 0 which is useless as a ColumnVec instance would not be created if its volume is zero;
4. remove col.set_toll() which is wrong and useless.

Other updates include,

1. make link generalized cost consistent in get_generalized_cost() in class Link and _update_generalized_link_cost(), and apply get_generalized_cost() in _update_generalized_link_cost();
2. use inheritance for some interfaces in classes;
3. add #define GUARD_PATH_ENGINE_H to path_engine.h;
4. remove implementations related to DLSim but keep read_zones() and read_demand_matrix();
5. introduce badges to README to better show the status.