finished the DiscreteEnvironment.py, hasn't validate
refine DiscreteEnvironment.py
fill in GetSuccessors and tested
fill in ComputeDistance and tested
fill in ComputeHeuristcCost and tested
add collision free constraint to the getSuccessor function, haven't validate yet
refine the DiscreteEnvironment.py and SimpleEnvironment.py, finish all the functions, and tested
finish HerbEnvironment.py, haven't validated yet
finished getSuccessors, haven't validated
finish the BFS and DFS
Merge branch 'master' into pap_dev
Parshad's code for planner updated