Using Unordered Map for dynamic obstacle validity checking

[JustinKottinger]

Using an unordered map for dynamic obstacle collision checking is not very clean. In addition, the current implementation does not account for robots with different propagation step sizes. A better solution would be to add a method to og::Path and oc::Path to return a state along the path at a given time. Not only will this resolve the aforementioned issue, it could possibly outperform the unordered_map, which averages O(1) complexity, with a vector lookup, which has truly O(1) complexity. Then, the collision checkers can use this method instead of relying on the map.