Fix exception when SmtInterpol produces a value like 1/3 for a variable
assignment (BigDecimal cannot represent it).
We now use ExtendedRational for it.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13795 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c