Improve naming and description of VCs

src/main/scala/leon/purescala/TreeOps.scala

src/main/scala/leon/purescala/TypeTreeOps.scala

src/main/scala/leon/verification/DefaultTactic.scala

src/main/scala/leon/verification/InductionTactic.scala

src/main/scala/leon/verification/VerificationCondition.scala

src/main/scala/leon/verification/VerificationReport.scala

src/main/scala/leon/xlang/XlangAnalysisPhase.scala