added a very basic implementation of 'sliced interpolation' to predicate analysis; the basic idea is to perform itp on a sliced path stemming from the original CEX; only works with SBE so far

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/branches/slicedItp@13899 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c