[WIP]: support for getting ik on flat ground - do not merge, not robust

[antoinefalisse]

@suhlrich this PR adds support to correct for a non-perpendicular checkerboard.

Algorithm in brief:

1. Using vertical speed of mid-hip to identify gait cycles
2. Assuming that vertical position of mid-hip at these times should match
3. Computing angle between X-axis and vector that goes from mid hip coordinated at first identified peak to mid hip coordinated at last identified peak
4. Rotating the data about z-axis
5. Computing vertical offset and correcting for it
6. Generating TRC
7. Running IK

Regarding the example. We can either not push it or either use a different one. It points to a private session now.