Use actual layer depths in radar_simulator.

```
radar_simulator uses zlev (local hgt_matrix) to calculate the depth of the 
layers, which is used to calculate attenuation by hydrometeors and gases. The 
actual depth is available in gbx%dlev, so a more accurate calculation can be 
done.
```

Original issue reported on code.google.com by `alejandr...@metoffice.gov.uk` on 16 Aug 2013 at 9:36
