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.
Rename module partition to partitioner.
This cultivates naming used in existing modules like extractor,
customizer, etc. - noun vs verb (word partition is both though).
There is no possibility until multiple-weights are implemented
to break tie in the contraction and the direct shortest path plugin
as duration is not computed during contraction.
This must be fixed after multiple-weights implementation
by using `std::tie(weight, duration)` pairs everywhere.
