CEX: Added CPA interface. If no refiner is used, the concrete CEX of the
first cpa that implements the interface will be used. Implemented for
value and smg analysis.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14502 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c