- `DistanceMatrix.from_addresses()` takes a variable number of
`Address` objects and creates distance matrix entries for them
- as a base measure, the air distance between two `Address`
objects is calculated
- in addition, an integration with the Google Maps Directions API is
implemented that provides a more realistic measure of the distance
and duration a rider on a bicycle would need to travel between two
`Address` objects
- add a `Location.lat_lng` convenience property that provides the
`.latitude` and `.longitude` of an `Address` as a 2-`tuple`
- the class stores the data of a distance matrix between all addresses
+ air distances
+ bicycle distances
- in addition, the "path" returned by the Google Directions API are
also stored as a JSON serialized sequence of latitude-longitude pairs
- we assume a symmetric graph
