SDAC - Export damage distributions per asset from DB to NRML

[Lars Butler]

Bug # 922506 adds the damage distribution models to the database.
Bug # 922508 adds the damage distribution models to the NRML spec.

The purpose of this bug is to add functionality to export damage distributions (per asset) from the database to NRML XML. The functionality should be callable from the command line (using bin/openquake --export). It should also be hooked into the SDAC calculator's post_execute() function (though the exporter should only be called if openquake is run with the --output-type=xml option).

There are two parts to this work package:
1) Write a NRML serializer to generate the dmg dist per asset NRML artifacts.
2) Write an export function which plugs into the existing oq-engine export framework to pull dmg dist per asset data from the database and pass them to the NRML serializer.