gadget.simulate update: 
- includes ability to read gadget models
- projects based on gadget.model objects
- possible multi-stock model 

known issues:
- pretty damn slow
- weight update is bonkers
- migration and tagging routines are not fully implemented

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgadget@229 edb9625f-4e0d-4859-8d74-9fd3b1da38cb