Make a special case for the situation when the hazard is known at the exact location of the sites

[Michele Simionato]

This happens when the sites are determined from the exposure, i.e. the risk calculation has a predefined exposure.
In that case the HazardGetterBuilder does not need to run a geospatial query and can be much more efficient.

[Michele Simionato (michele-simionato)]

https://github.com/gem/oq-engine/pull/1510

[Michele Simionato (michele-simionato)]

As an example I run a Scenario Damage calculation for Portugal, a case with 268,056 assets located on 4,050 sites. The asset->site association operation with the geospatial queries took 1,142 cumulative seconds. With the improvement it takes only 380s, exactly a speedup of a factor of 3. Not bad,