Make BasicProverEnvironment generic
such that the method push(BooleanFormula) can be added to it
(InterpolatingProverEnvironment needs it to have a generic return value).
This allows to unify some code that should work for
both ProverEnvironment and BasicProverEnvironment.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14918 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c