add recipe result message string

[o-smirnov]

I can think of some recipes (e.g. TRON) where a human-readable summary of results at the end of a (successful) run would be useful. Suggest something like a summary_message property (subject to the usual substitutions and formula evaluations) which can be dumped to the console at the very end of a run.

[o-smirnov]

Also an idea, maybe it should be just a regular output with a "magic" name, and if such an output exists, stimela will print it automatically. Or maybe add a property to the schema indicating that the output should be printed.