Add Grid.gridify() constructor

- 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

setup.cfg

@@ -170,6 +170,9 @@ per-file-ignores =
 # Source: https://en.wikipedia.org/wiki/Cyclomatic_complexity#Limiting_complexity_during_development
 max-complexity = 10
 
+# Allow more than wemake-python-styleguide's 5 local variables per function.
+max-local-variables = 8
+
 # Allow more than wemake-python-styleguide's 7 methods per class.
 max-methods = 12