Re-factor code into 'core' library

[chadrockey]

Soon it would be good to re-factor the core functions out of each application and into a 'graft_core' library. I'm not sure how strict it will be, probably just helper functions on eigen matrices.

I probably won't do this until after I get a GPS example. I think that 'robot_pose_ekf', 'attitude', and 'GPS' should be enough examples to have a good idea of what the core library should look like.

[mikeferguson]

@trainman419 @chadrockey Just a heads up that I should have a pull request in a day or two that addresses most of this. Basically, after reviewing the code, 95% of it is shared among all three filters, and so I've implemented a GraftUKF class that is templated on SIZE, and then the velocity, attitude and altitude filters are template specializations.