[Front] Add comments; update [gs]etters in Answers."
[Front] Update comments
[Front] Delete redundant interfaces
[Front] Make Process return type covariant

Request is now a generic : Request<TAnswer>, where TAnswer is an Answer.
Process returns a TAnswer.
Subclasses of Request  are not generic and set TAnswer to the
appropriate answer type.
This allows the covariance of the return type of  Request.Process
method.