determining unit sandbox URL *before* it start running, otherwise file
pre-staging won't happen. added read-only properties to pilot and cu:

pilot.sandbox
  # returns something like sftp://alamo.futuregrid.org/home/oweidner/sagapilot.sandbox/pilot-530c34be14b25bb6399994ed

cu.sandbox
  # returns something like sftp://alamo.futuregrid.org/home/oweidner/sagapilot.sandbox/pilot-530c34be14b25bb6399994ed/unit-530c34bf14b25bb6399994ef

useful for inspection and debuggin, and, well, data staging.
added target URL to transfer func