Leon can handle abstract examples if the flag `keepAbstractExamples` is set to...

Leon can handle abstract examples if the flag `keepAbstractExamples` is set to true in ExamplesFinder.
(It means that examples will not be enumerated if they have free unassigned variables, but kept as it.)
input/output examples can now match only a part of the inputs, but the output is still compulsory
This is now used in StringRender.scala.
The functional example works.