[Cs3] underlying containers for container adapters

[Mikhail Nesterenko]

CS3 students,

Not every sequential container may be used as an underlying container
for every sequential adapter. However, the possibilities and defaults
are "reasonable" with respect to container use and implementation.

stack - underlying may be vector, deque and list, deque is default
queue - underlying may be deque and list, deque is default
priority_queue - underlying may be vector and deque, vector is
	       default.

Thanks,
-- 
Mikhail