Added a QA test for purely population case
Removed empty tags from the exposure
Removed the old mixed test
Added parameter ignore_missing_costs
Merge branch 'master' of github.com:gem/oq-risklib into scenario-population
Missing costs are considered zeros
Used masked arrays to manage missing costs
Fixed some tests
Initial work on the classical_damage calculator
Moved a comment in the right place
Merge pull request #104 from gem/classical_damage

Initial work on the classical_damage calculator
Merge pull request #103 from gem/scenario-population

Added parameter ignore_missing_costs
Merge branch 'master' into classical-risk