Use ccdVisit table `skyNoise` to compute DM image depth

Since it's in DN, I guess we want to do
```python
D_sky = zeroPoint - 2.5*np.log10(5*skyNoise)
```
What kinds of numbers does this give? Let's plot `D_sky` against `D_sigma` where the latter is our simple estimate based on the median flux uncertainty across all DIAsources in the visit.