prevent NaN generation when gravity is (0, 0, 0) and we switch from body is excluded to body is not excluded in gravity force appliction.  This is necessary as the rest of the code skips updates when gravity is special cased to zero.
Merge pull request #33 from hsu/simbody-3.3

Prevent NaN generation when gravity is (0, 0, 0) and a previously-excluded body has gravity re-enabled. This needs to be merged into the master branch also.