Add in compensation for Mx moments due to belt acceleration.

[moorepants]

Now that we have a stationary V-gait we do not need the inertial compensation with the accelerometers (although it would still help, once we get better accelerometers).

But we do need to to the compensation for the belt/roller acceleration. Sandy has found a linear relationship between Mx (Nm) and the acceleration (m s^-2). Theoreticaly, no other force plate signals should be affected by belt acceleration. Sandy can probably confirm that with her data.

I would suggest to do a 6 Hz zero-lag filter on the belt speed signal (make sure it's exactly the same filter that is used for all other signals), then do a 3-point zero lag derivative estimation (xdot_i = (x_i+1 - x_i-1)/(t_i+1 - t_i-1)) to get the acceleration. Multiply by the correction coefficient, subtract result from Mx and that's it.

If you use filtered velocity, you should correct after filtering the force plate signals, otherwise the correction signal gets filtered twice. Since all these processing steps are linear, I suppose you can use unfiltered velocity and correct before filtering the force plate signals. The order does not matter. The only thing is, if you look at a corrected but unfiltered Mx signal, it may look much noisier than the original Mx signal because it has correction related to a possibly noisy belt acceleration signal. At the end it's all the same, it only matters if you want to inspect these intermediate results.

[moorepants]

This should be a method on the DFlowData class and there should be a flag in the meta.yml file which says whether the belt speed is constant or not in the run. Finally, there should be a file (and event?) that has unloaded data which the compensation can be computed from. This should all follow a similar pattern as the inertial compensation due to pitch and sway.