Reworked mechanism for ignoring accept/reject during transitions so that 
when processing candidates accept/reject is not ignored during an 
observation transition (only a source transition).  Otherwise the behaviour 
is the same, with further transitions being ignored, and accept/reject
being ignored for during any transition while processing reals.
Made CandidatesWorkUnit create its writer lazilly.  Cleaned up some test 
cases that had cross-interactions.
Removed freeze feature from WorkUnit and StatefulCollection because 
re-implementation of the TransAckUIModel doesn't use it.