Don't Throw Error for Empty Requests

- add empty group signature
- prettify log output (spaces)
Allow clock systems for domain offsets

If the domain offsets are ordered in a clock system instead of
starting with (0,0,0) at file (0,0,0), this patch allows to read
them.

First, the file with the (0,0,0) has to be found. Afterwards,
the same divide&conquer algorithm looks for the top-left corner,
but only in the "upper" or "lower" volume, seen from the (0,0,0)
file.

At the end, the real read algorithm will walk over an "optimal"
number of files for requested planes. If no plane was requested
but a general volume, the algorithm (should) still work but is
less efficient if one is not looking for the full domain
(same behaviour as before).