SDAC - support interpolation between the lowest IML and the noDamageLimit

[Giuseppe Vallarelli]

Problem:
Some values in the ground motion fields could be smaller than the lowest intensity measure level in the fragility model which means, the calculator would not have enough values to interpolate the probability of exceedance for each limit state.

If the attribute noDamageLimit is defined: In this case, OpenQuake will estimate the probabilities of exceedance by interpolating between the lowest IML from the fragility model and the noDamageLimit. For instance, let's consider the fragility model I already gave you, more specifically the fragility functions set for RC, whose lowest IML is equal to 0.1. Let's now assume that the noDamageLimit has been set to 0.05 and two IMLs from the GMF equal to 0.01 and 0.075 are provided. For the first case of 0.01, the value is lower than the noDamageLimit (0.05) which means OpenQuake will assume that the probabilities of exceedance are equal to zero (same as Option 1). In the second case (IML = 0.075) you have to interpolate assuming PoE = 0 for the noDamagelimit. In this case you would obtain the following PoEs for each limit state:
        - LS1: PoE = 0.00015
        - LS2: PoE = 0.0005