Cost Distributions become Histograms

src/main/scala/leon/refactor/RepairCostModel.scala

src/main/scala/leon/refactor/Repairman.scala

src/main/scala/leon/synthesis/CostModel.scala

src/main/scala/leon/synthesis/Distribution.scala

src/main/scala/leon/synthesis/Histogram.scala

src/main/scala/leon/synthesis/PartialSolution.scala

src/main/scala/leon/synthesis/Problem.scala

src/main/scala/leon/synthesis/graph/DotGenerator.scala

src/main/scala/leon/synthesis/graph/Graph.scala

src/main/scala/leon/synthesis/graph/Search.scala

src/main/scala/leon/synthesis/rules/Cegis.scala

src/main/scala/leon/synthesis/rules/GuidedCloser.scala

src/main/scala/leon/synthesis/utils/ExpressionGrammar.scala

src/main/scala/leon/utils/InterruptManager.scala