Fragility model using structure dependent IMT

[matley]

Similarly to https://bugs.launchpad.net/openquake/+bug/1149270 we need to support structure dependent IMT in the scenario damage case.

In order to do this, we need to change the schema of fragility models. E.g.

    <fragilityModel format="discrete" imlUnit="g">
        <description>Fragility model for Pavia (discrete)</description>

        <!-- limit states apply to the entire fragility model -->
        <limitStates>
            minor
            moderate
            severe
            collapse
        </limitStates>

        <!-- fragility function set, each with its own, distinct taxonomy -->
        <ffs noDamageLimit="5">
            <taxonomy>RC/DMRF-D/LR</taxonomy>
            <IML IMT="MMI">7 8 9 10 11</IML>

            <!-- fragility function in discrete format, one per limit state -->
            <ffd ls="minor">
                <poEs>0.0 0.09 0.56 0.91 0.98</poEs>
            </ffd>
            <ffd ls="moderate">
                <poEs>0.0 0.00 0.04 0.78 0.96</poEs>
            </ffd>
            <ffd ls="severe">
                <poEs>0.0 0.00 0.00 0.29 0.88</poEs>
            </ffd>
            <ffd ls="collapse">
                <poEs>0.0 0.00 0.00 0.03 0.63</poEs>
            </ffd>
        </ffs>
</fragilityModel>