Galaxy dump

This adds the possibility to create text dumps of the generated galaxy from the command line

**Synopsis**

```pioneer -gd filename [radius] [sx,sy,sz]```

**filename** (mandatory): File to dump to or ```-``` to dump to ```stdout```.
**radius** (optional, default 4): Defines sizes of cube to dump.
**sx,sy,sz** (optional, default 0,0,0): Center sector of cube to dump.

A cube of sectors centered at ```(sx,sy,sz)``` with the given radius is dumped, i.e. all sectors from ```(sx - radius, sy - radius, sz - radius)``` to ```(sx + radius, sy + radius, sz + radius)```.

I usually use it to verify that code changes did not logically change the generated universe, e.g
```pioneer -gd - 20 -82,41,0 | md5sum``` (actually I run several of them in parallel from a script and dump the md5sums to a file).