SessionBasedScript requires that TaskCollections have an attribute output_dir

[riccardomurri]

Currently, the SessionBasedScript class assumes each task given to it has a .output_dir
attribute, including TaskCollection instances and other tasks where .would_output == False.

Here is an example of the traceback::

exercise_E.py: [2016-11-22 16:43:07] CRITICAL: AttributeError: 'TwoStageWorkflow' object has no attribute 'output_dir'
Traceback (most recent call last):
  File "/home/rmurri/gc3/gc3pie/gc3libs/cmdline.py", line 471, in run
    return cli.app.CommandLineApp.run(self)
  File "/home/rmurri/.virtualenvs/gc3pie-dev/local/lib/python2.7/site-packages/cli/app.py", line 245, in run
    return self.post_run(returned)
  File "/home/rmurri/.virtualenvs/gc3pie-dev/local/lib/python2.7/site-packages/cli/app.py", line 241, in run
    returned = self.main(*args)
  File "/home/rmurri/gc3/gc3pie/gc3libs/cmdline.py", line 1279, in _main
    self.process_args()
  File "/home/rmurri/gc3/gc3pie/gc3libs/cmdline.py", line 950, in process_args
    self._add_new_tasks(new_jobs)
  File "/home/rmurri/gc3/gc3pie/gc3libs/cmdline.py", line 1018, in _add_new_tasks
    self._fix_output_dir(task, task.jobname)
  File "/home/rmurri/gc3/gc3pie/gc3libs/cmdline.py", line 1026, in _fix_output_dir
    task.output_dir = task.output_dir.replace('NAME', name)
AttributeError: 'TwoStageWorkflow' object has no attribute 'output_dir'

The problem seems to be that _fix_output_dir tries to replace/patch
the result of task.output_dir unconditionally.

[riccardomurri]

The easy workaround is just to initialize every TaskCollection
instance with a (possibly bogus) output_dir attribute::

def new_tasks(self, extra):
  # ...
  return [
    TwoStagedTaskCollection([], output_dir="not-used")
  ]