Allow arbitrary paths for the restart file

[G-071]

Currently, Octo-Tiger requires restart files to be in the same directory, no matter what value is given with the --datadir parameter. This should be fixed (or a new parameter added) to allow arbitrary locations for the restart files.

Note, sometimes during distributed runs Octo-Tiger currently just hangs/deadlocks when the restart file is not found (most likely due to it being in the wrong directory) instead of printing the proper error message. This should be fixed, too!