Integrate inertial compensation into DFlowData and 2D inverse dynamics into WalkingData.

This adds a Python wrapper using oct2py to the inertial compensation code. Here is the TODO list to finish this PR:

- [x] low pass filtering of data frame columns
- [x] delsys time delay adjustment
- [x] wrapping of the inertial compensation function
- [x] update the command line program with new options (filtering, compensation, etc)
- [x] get test coverage close to 100%
- [ ] Add plot function that compare pre-compensation to post-compensation.
- [x] Deal with the low pass filter failing on SciPy 0.9.0.
- [ ] Update documentation.
- [x] Add 2D inverse dynamics to WalkingData.
- [ ] Modularize the filtering. Note that the data should be filtered before computing the inertial compensation forces.