several changes

In Crossing.CS:
-added attributes _entered and their properties;
-added attribute probWtoE and its property (we forgot about this one!)
-changed reset() so it clears all cars in the crossing's streets.
In Car.cs:
-added a Crossing attribute and its property
-Made all attributes private
-made all properties public
-deleted doubled comments
In Grid.CS:
-implemented timeTick() method (still has to be tested)
-implemented reset() method