Linearize about proper pitch angle.

[moorepants]

I can't remember if it is acceptable to linearize about pitch=0.0.

https://github.com/moorepants/HumanControl/blob/master/whipple_pull_force_abcd.m#L81

If this model was derived such that pitch=0.0 for the nominal config, then it is correct. If it was not, then it may not be correct.

But the eigenvalues of the Benchmark bicycle created with this code is probably correct.

Something worth checking.