Add stop access and egress times

[jfbischoff]

Some public transport stops are more accessible than others.
One typical example: Accessing a metro stop is often much more cumbersome than accessing a tram stop, because one has to go down several metres.
So far, accessing stops by usually teleported walk routes, this could not be taken into account. Thus we could see in our models that taking trains or metros for very short trips is more popular than it should be if there is competing ground level public transport.
This PR allows the introduction of access and egress times for each stop facility (not including transfers between public transport, as these allow an explicit definition of a transfer time).

Depending on your pt data source this may or may not be useful. The Swiss GTFS essentially treats all stop points as single stops, the Berlin GTFS on the contrary does not, i.e., a combined tram and metro stop is treated as a single stop.

Access and egress times can be set to different values, which may be handy if one considers modeling long distance trains (e.g., TGV has a boarding time of 5 to 2 minutes) or even airplanes.

Access and egress times can be set via Attributes on the facility, some Convenience methods are provided in TransitScheduleUtilities.