Robustify IMU integration

[nashj]

Feature Request to protect against IMU dropout.

Ideas:

• Provide a list of IMUs to SERPENT, and have SERPENT build a CombinedIMUFactor for each. The ordering of these IMUs would indicate which should be preferred for motion distortion correction and as an initial guess for registration/stereo modules.
• Cope with all IMUs failing, by detecting when no IMU messages have arrived for a certain amount of time, and then adjusting the logic of motion prediction and backend optimisation accordingly.