The rupture seed is now visible in the XML file

This is just a debugging help. When comparing the GMFs produced by to different versions of the engine, it is vital to check if they have been generated by the same ruptures *with the same seeds*. The easiest way to check that is to compare the exported SES files. This PR adds the seed information as a comment into the SES file. Since it is just a comment, nothing else changes.
Here is an example of the output:

```xml
    <stochasticEventSet id="2" investigationTime="50.0">
      <rupture id="smlt=00|ses=0002|src=3|rup=004-01" magnitude="6.15" strike="45.0" dip="30.0" rake="90.0" tectonicRegion="Active Shallow Crust">
        <!--rupture seed=803676216-->
        <planarSurface>
          <topLeft lon="85.6735632444" lat="27.8857387" depth="5.73985506245"/>
          <topRight lon="85.7655277775" lat="27.9670291961" depth="5.73985506245"/>
          <bottomLeft lon="85.7266589672" lat="27.8388333978" depth="10.0"/>
          <bottomRight lon="85.8186235003" lat="27.9200886483" depth="10.0"/>
        </planarSurface>
      </rupture>
   ...
```

This fix helped debugging https://bugs.launchpad.net/oq-engine/+bug/1329674