- add new ORM models `ReplaySimulation` and `ReplayedOrder`
to store the data generated by the routing simulations
- add migrations script to create the corresponding database tables
+ create "replay_simulations" and "replayed_orders" tables
+ add missing check constraints to "orders" table
+ add unique constraint to "orders" table to enable compound key
+ drop unnecessary check constraints from the "orders" table
- add tests for the new ORM models
+ add `simulation`, `replayed_order`, `make_replay_order()`, and
`pre_order` fixtures
+ add `ReplayedOrderFactor` faker class
- fix some typos
- add Grid, Pixel, and AddressPixelAssociation ORM models
- each Grid belongs to a City an is characterized by the side_length
of all the square Pixels contained in it
- Pixels aggregate Addresses => many-to-many relationship (that is
modeled with SQLAlchemy's Association Pattern to implement a couple
of constraints)
- create `*Factory` classes with fakerboy and faker that generate
randomized instances of the ORM models
- add new pytest marker: "db" are the integration tests involving the
database whereas "e2e" will be all other integration tests
- streamline the docstrings in the ORM models
