[FEATURE REQUEST] Isochrones

[LdDl]

Is your feature request related to a problem? Please describe.
Isochrones in terms of GIS - https://wiki.openstreetmap.org/wiki/Isochrone
This is nice instrument for transport accessibility analysis.

Describe the solution you'd like and provide pseudocode examples if you can
Implement shortest path finding algorithm with cost restristions (issue is #7 )
Wrap implemented function inside something like this (pseudocode):

Isochrones(source, max_cost) []int64{
    targets := graph.GetAllVertices
    isochrones := map[int64]bool
    answer = []int64
    for target in targets {
        if target == source ||  single_vertex is in isochrones {
             continue
        }
        _, single_isochrone_path = ShortestPath(source, target, max_cost)
        for single_vertex in single_isochrone_path {
           if single_vertex is not in isochrones {
                isochrones[single_vertex] = true
                answer = append(answer, single_vertex)
           }
        }
    }
    return answer
}

Describe alternatives you've considered and provide pseudocode examples if you can
There is alternative solution:
Just iterate all neighbor vertices on each step and collect them while estimated cost < max_cost
It's just a bit modified (with provided max cost) BFS - https://en.m.wikipedia.org/wiki/Breadth-first_search

Additional context
Nope

[LdDl]

See #9