This change takes the existing typedefs for weight, duration and
distance, and makes them proper types, using the existing Alias
functionality.
Primarily this is to prevent bugs where the metrics are switched,
but it also adds additional documentation. For example, it now
makes it clear (despite the naming of variables) that most of the
trip algorithm is running on the duration metric.
I've not made any changes to the casts performed between metrics
and numeric types, they now just more explicit.
This PR improves routing results by adding support for snapping to
multiple ways at input locations.
This means all edges at the snapped location can act as source/target
candidates for routing search, ensuring we always find the best route,
and not the one dependent on the edge selected.
