- `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 purpose of this constructor method is to generate all `Pixel`s
for a `Grid` that have at least one `Address` assigned to them
- fix missing `UniqueConstraint` in `Grid` class => it was not possible
to create two `Grid`s with the same `.side_length` in different cities
- change the `City.viewport` property into two separate `City.southwest`
and `City.northeast` properties; also add `City.total_x` and
`City.total_y` properties for convenience
- the old `UTMCoordinate` class becomes the new `Location` class
- its main purpose is to represent locations in both lat-long
coordinates as well as in the UTM system
- remove `Address.__init__()` and `City.__init__()` methods as they
are not executed for entries retrieved from the database
- simplfiy the `Location.__init__()` => remove `relative_to` argument
- the class is a utility to abstract working with latitude-longitude
coordinates in their UTM representation (~ "cartesian plane")
- the class's .x and .y properties enable working with simple x-y
coordinates where the (0, 0) origin is the lower-left of a city's
viewport
