Documentation of IMUFactor

[Frank Dellaert]

@djensen3 (cc @lucacarlone, @varunmurali1, @KrunalChande),

I thought one of your tasks was documentation :-)

Sure would love to know what all the arguments are here:

    /** Constructor */
    ImuFactor(Key pose_i, Key vel_i, Key pose_j, Key vel_j, Key bias,
        const PreintegratedMeasurements& preintegratedMeasurements, const Vector3& gravity, const Vector3& omegaCoriolis,
        boost::optional<const Pose3&> body_P_sensor = boost::none) :

[Frank Dellaert]

@djensen3 did you see this?

[David Jensen]

I've added some comments here (and to CombinedImuFactor.h)