Add newProverEnvironment, newProverEnvironmentWithInterpolation,
and newOptEnvironment to the FormulaManager interface.
This simplifies FormulaManagerFactory,
because it now does not need to know how to create
these environments for each of the solvers.
All the implementing classes of these interfaces
can now be made package-private.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14906 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c