Fix/moving window

This pull fixes some moving-window related issues.

### Empty data sets

Don't throw an assert if we request to read a (x,y,z) data set with x||y||z==0. This indeed *is* the correct meta information for a data set that was intended to be written as *empty*.
The corresponding *hdf5* response is a (1,1,1) data set without information.

Mixing this up with a *real* (1,1,1) data set can be avoided, since we *know* that we already *requested* the data set x*y*z=0 .
```c++
const bool empytRequest = ( data_size.getScalarSize() == 1 &&
                            client_domain.getSize().getScalarSize() == 0 );
```

### Test intersection algorithm

The test intersection algorithm for fast slice writing was not tested for moving windows yet. This pull fixes an issue about forgetting the bottom plane.

For simulations with moving windows, the test intersection does not by default find the TOP-LEFT corner of the
intersected plane. Of course it is not important, if the first intersection is found at the corner or *somewhere* in the plane.
This fix helps the algorithm, so it still runs through the *full plane* after the intersection *somewhere* within the plane was found.