Introduce a table with the sites of interest for a given hazard computation

[Michele Simionato]

This is needed for the risk and in general it is useful even for the hazard. There is actually already a site table in the engine, called site_data: however, it is the transpose of the needed one, instead of storing (site_id, location, parameters) it stores arrays with the coordinates and the parameters. The ticket requires fixing the site_table data to be useful for the risk. It also asks to move it from the htemp schema into the hzrdi schema, since it becomes a persistent table.

For more info about the motivation, look at https://docs.google.com/a/openquake.org/document/d/19q-A6AJCMXflMfp9gKmtCqF2fk2EngfQvoQy7O6xUHQ/edit?usp=sharing

In short, the final goal is to remove the expensive aggregations by location
which are needed for the risk.

[Michele Simionato (michele-simionato)]

Related pull requests:

https://github.com/gem/oq-hazardlib/pull/123
https://github.com/gem/oq-engine/pull/1192