Support for non structural losses

[matley]

The user should be able to specify multiple vulnerability files instead of a single one. E.g.

structural_vulnerability_file=foo.xml
occupants_vulnerability_file=bar.xml
contents_vulnerability_file=baz.xml
non_structural_vulnerability_file=foo.xml

These 4 params substitute the old one ("vulnerability_file"). It is ok if the user specify only one, two or three of them. In that case only the related analysis will be run.

Additionally for the scenario calculator a new parameter, time_event, is required only if contents_vulnerability_file is specified:

time_event = day

It is used to get the proper occupants value from the exposure file.

If an exposure lacks some value (e.g. no occupants but occupants_vulnerability_model has been specified) a warning during the calculation will be raised.

For each kind of output (e.g. LossCurve) we will end up with a final output for each vulnerability model specified.

Therefore, if previously we had x loss curve set, we will end up with (at most) 4 x loss curve set (one for each vulnerability file specified).

[matley (matley)]

https://github.com/gem/oq-engine/pull/1168