Allow calling driver.py from another python script

To automate numerical experiments, make it possible to call the main routine in driver.py from a different python script.
Remove omega2 from namespace after usage

If the driver.py script is called multiple times from a different python script, the global PyOP2 constant raises a NonUniqueNameError. To avoid this, remove from namespace after usage.
Ignore all files in output directory.
Merge branch 'allow_call_from_other_script'