Merge pull request #102 from javierhe/master

New factory for loading scenario from file
Addiditons for import scneario all in one go.
Merge branch 'master' of github.com:calpath/model-objects
Fixing in case of null sensor lane numbers.
Changes to model objects to account for unset demand MO attributes.
Merge pull request #103 from mnjuhn/master

Changes so that entire scenario can be inserted into DB from XML file.
Add method to get the current FD in the profile for a specified time
Move density to the base class, in order to get it serialized