Support asset category = population

[matley]

We need to enhance the exposure getter (implemented in openquake.engine.db.models.AssetManager#get_asset_chunk) in order to fetch properly the occupancy data depending on the asset category.

When asset category is population the number of people is stored into the number_of_units field of each Asset.

If asset category is not population then we read the Occupancy table to get the average data.

If a time_event is specified in the calculation configuration then we read only the rows of the Occupancy table related to the given time_event value.

[matley (matley)]

https://github.com/gem/oq-engine/pull/1196