ConfigSequence.py

Given that the first iteration of the configuration block design
invoked criticism of the "reference" design for tracking containers I
now changed it, so that instead of first allocating references to
determine what the intermediate containers are, I just run through the
entire configuration twice.  The second run-through then can rely on
what it learned in the first run-through to determine what will happen
further down in the sequence.

This requires routing a number of calls through the ConfigAccumulator
object, but it definitely makes writing the actual blocks more
straightforward, and it should also allow for more flexibility if we
need to redesign the ConfigAccumulator in the future (which also
became simpler with this change).