[Gzz] Why the mirror test fails

[Benja Fallenstein]

Hi,

The mirror test currently fails because MasterImpl, after getting the 
changes from its space, applies them back to the space; this assumes 
that checkpointDelta() reverses the changes it returns. However, the 
tests assume the opposite:

>     conn(6,7)
>     conn(0,1)
>
>     self.checkpointDelta()
>
>     # 0-1 6-7
>     asRank([0,1], [6,7])


(from dim.py). Tuomas: Please decide what this should be and javadoc 
gzz.diff.DeltaSpace accordingly! Then, it should be easy to fix the test.

- Benja