Restarting from Old Enzo Datasets

[dcollins]

Ensure Enzo1.5 can restart from old datasets with zero hassle from the user. Things I've found so far:

1. Task = ? line in hierarchy:
   1. Check for the lines existence.
   2. If it isn't there, set some reasonable default (probably round-robin over grids)
2. Make sure this works with old Packed formats.