Add initial python code for writing body force sections
Add logic to use unassociated attribute as default body force

Applied to any/all domain sets that not associated to same body force type
Minor fixes for body-force output

* Updated logic to convert domain set name to id
* Only use 1 unassociated attribute for given body force type
Extend body force logic for compatibility with current SMTK

Cannot output "default" (not associated) body force attributes
Need SMTK commit b27ef03 (26-Aug-2014) for default body forces
Changing HeatSource item to be consistent for Hydra.