a new method to investigate selected events

A prefix method could be used to access directly given events. This could be seen as a kind of TTree::Scan() but running the event loop for selected events in actual executables. This can be useful to tests certain loops for extreme events without submitting a whole workflow.

Something as the following:

scan cmd input/ output/ [args...]

Different options can be considered:

• provide the condition from the command line
• provide a prompt in the execution to provide the conditions

Some tweak in Darwin::Tools::Looper is expected in any case.