Add let expression in VeriT format

Fix issue #68
Merge remote-tracking branch 'upstream/develop' into develop
Merge remote-tracking branch 'afellner/develop' into correction2
Add existsAmongAncestors method to ProofNode
Improve existsAmongAncestors for speed
Merge remote-tracking branch 'existsAA/feature/existsAA' into develop
Implement suggested changes in BWSt