renamed units -> number_of_units
exposure documents updated
refactored get_asset_chunk to make it more testeable
improved doc
Merge pull request #1196 from matley/number-units

renamed units -> number_of_units. Support for asset_category == "population"
Merge branch 'master' of https://github.com/gem/oq-engine into update-risk-demos
Merge pull request #1197 from matley/update-risk-demos

Update risk demos
Merged from master
Fixed all the tests