Do not write unused VDS files in the first place

When, in artemis, we have a large grid, there are more than two datasets linked in the nexus files, and for the second nexus file, the first dataset may not be used. In that case,

https://github.com/DiamondLightSource/python-dlstbx/blob/main/src/dlstbx/swmr/h5check.py

fails when running

```python
plist = dataset.id.get_create_plist()
...
vspace = plist.get_virtual_vspace(0)
frames = vspace.get_regular_hyperslab()[3][0]
```

because that dataset has no corresponding slab of the vds.

I would like it if completely unused datasets are not linked in the nexus file.