Added gravity model. Started cleening up the code a bit.
tree03e2fc03c7e233f9dde9d167850b69afe7077ced
-rw-r--r-- 64 AUTHORS
-rw-r--r-- 32400 License.txt
drwxr-xr-x - examples
drwxr-xr-x - src
drwxr-xr-x - tests