Details about IMU Integration Covariance

[varunagrawal]

Feature

@lucacarlone and I need to document what the Integration Covariance is and its role in the ImuFactor.

[dellaert]

Please be careful: ImuFactor.pdf is about my new Tangent-space formulation. The Forster et al. papers are the reference for the manifold pre-integration version. I don't mind expanding and re-capping that math in this file as well, but that is a more substantial undertaking.

[mattking-smith]

@lucacarlone or @dellaert I haven't come across the paper detailing the math on the Integration Covariance from Forster. Do you have specific paper in mind? So far I am able to calibrate my IMU setup with an Alan Variance (AV) analysis, but I essentially "trial and error" the integration covariance as the AV analysis doesn't determine this information.