fix 893
Merge pull request #898 from mbernasocchi/inasafe-893

[HOTFIX] Fix #893 - aggregation produces zeros for results on volcano population.
Fix use native zonal stats checkbox
Merge pull request #899 from mbernasocchi/fix-native-stats-checkbox

Fix use native zonal stats checkbox
[#703 Wizard] small fixes
Merge branch 'develop' of github.com:AIFDR/inasafe into develop