Why not to have a time window for the pickup and for the delivery steps?

[garciparedes]

Is your feature request related to a problem? Please describe.
In the current implementation, there is only one time window (earliest and latest attributes) stored in the Trip object. In the Cordeau's Dial-a-Ride model there can be two types of time windows, the inbound and the outbound tasks so in the current implementation there is also a inbound boolean attribute that is in charge to give the proper meaning to the earliest and latest attributes.

Describe the solution you'd like
An easy way to clarify concepts and be more flexible can be to have two time windows for each Trip. One which represents the pickup's time window and another which represent the delivery's time window. An interesting naming scheme can be pickup_earliest, pickup_latest and delivery_earliest, delivery_latest.

Describe alternatives you've considered
One possible alternative (or extension) that could be implemented through a future issue can be to encapsulate the respective time window and position (for pickup and for delivery) in to a new object.

Additional context
This issue is strictly related with #48.