Add an option inside the SMT solver package
to approximate floating-point numbers with rationals,
similar to how we can approximate bitvectors with integers.
The advantage of this is that even in case we do not want to use real
floating-point arithmetic, the calling code can still keep using the
same interfaces and methods,
and it is transparently replaced.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14529 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c