Support non-structural losses

Currently, risk calculators only consider structural costs and losses. The user should be allowed to provide an exposure containing costs not only for the physical structure of an asset but also for the contents (e.g. equipment), non structural components (e.g. water plumbers) and occupants as well. Then, given in input distinct vulnerability models for each type of costs the risk calculators will compute their standard outcomes for each type of costs. E.g.

structural_vulnerability_model=foo.xml
contents_vulnerability_model=bar.xml
occupants_vulnerability_model=baz.xml
non_structural_vulnerability_model=foz.xml

For the last one, the exposure schema needs to be modified to include the value of non structural components.

In the case of computing losses for occupants, the user should be able to specify multiple occupants value for an asset each one identified by a time value, e.g.

<occupants description="day">300</occupants>
<occupants description="night">30</occupants>
<occupants description="transit">70</occupants>

In the case of the scenario calculator a specific value could be choosen. In the other calculators, the mean value is used.

Implementation suggestion: In order to avoid moving the same hazard data we should avoid distributing the computation over kind of losses.