Support for the Use Case of Determining What was a Task Doing on What Data (MIMD)

It is better to view these 3 changes as a whole. For instance, 5034ce9 makes little sense on its own because currently all calls to <code>PyMW_Task.__init__</code> pass a string as the <code>exec</code> argument (which makes the check itself useless anyhow) but after e6e83eb it is evident that a check on the <code>exec</code> argument is needed. Finally, eb27131 justifies all the reworking on the <code>_exec</code> attribute by exposing it through a getter.