provide checked form of vertex/edge property access method

[zhuoqiang]

in stl, there are both checked and unchecked method to access the value from container

vector[0]; // unchecked, may UB
vector.at(1); // checked, may throw

map["key1"]; // auto add missing key
map.at("key2"); // checked, may throw

graph as a container, however, does not provide a checked method to access the vertex/edge property using vertex/edge id

graph[vertex_id]; // unchecked, may UB
graph[edge_id]; // unchecked, may UB

How about providing a checked access method that throw exception in case of missing id. Also helps to keep the API consist with STL containers

graph.at(vertex_id); // unchecked, may throw
graph.at(edge_id); // unchecked, may throw

and in general, not sure if I missed it but could not find API to check if the id is valid or not:

graph.contains(vertex_id); // not sure if it exists