Standardized all impact function population impact. (issue #910)
Raise an exception when adding a duplicate impact function name.
Merge pull request #1090 from cchristelis/develop

Standardized all impact function population needs. (issue #910)
better computations in categorizing values
Merge pull request #1092 from cchristelis/feature/handle_duplicate_class_names

handle duplicate class names
merged from upstream
Merge branch 'develop' of https://github.com/AIFDR/inasafe into proposed_computations_categorised_hazard_population
no hazard areas are not included in low category