add allow_all option to script scanner recognized experiment headers

[fanmingyu212]

See @aransfor comments in #226 (comment). This option will allow an experiment to be run concurrently with all other experiments.

The default value for this option (when it is not present in an experiment header) should be False. When its value is False, it should read from allow_concurrent to get a list of experiments that can run concurrently with this experiment.