Deal with discrete fragility functions given in XML and not CSV files, using the OpenQuake libraries

The objective is to change the losscalculator to deal with discrete fragility functions given in XML and not CSV files, using the OpenQuake libraries.
I will be using the class FragilityModel from the OpenQuake risklib. This task is a part of the issue 378 in the collaboration repository.

#14 (closed) will also be covered in this issue.