SensorSet factory method
Merge branch 'master' of github.com:calpath/model-objects
InitialDensitySet MO
Merge pull request #72 from javierhe/master

QA fixes
InitialDensitySet MO fixes
Merge pull request #73 from javierhe/master

InitialDensitySet MO fixes
Merge branch 'master' of https://github.com/calpath/model-objects