Smart retrieval of the exposure in a risk calculation

[Michele Simionato]

I propose the following rules for the risk_calculation.exposure property:

1. if the `preloaded_exposure_model_id` parameter is set in job_risk.ini,
   use it
2. if an exposure_file parameter is defined in job_risk.ini, use it
3. if an exposure was used in the hazard job, use it
4. if no exposure is found, return None

Then, by running

   openquake --rr job_risk.ini --hazard-job-id JOB_ID

if would be possible to avoid reloading the exposure if it was already loaded in the hazard. For this feature to work, we
must add a flag --hazard-job-id. At this point, it might be a good idea to remove --hazard-calculation-id, since it becomes useless.

[Michele Simionato (michele-simionato)]

https://github.com/gem/oq-engine/pull/1508