Added an odometry prior callback function

[bobbyshashin]

Hi Humphrey,

Could you please take a look at this? I'm not sure whether it's the correct way to do it.

[Humhu]

What's the motivation behind having a different odometry topic for the priors, versus using the existing odometry callback?

[bobbyshashin]

Actually that is the part where I don't understand the factor graph theory behind very well. So I should either use the pose, or integrate the velocity, depends on whether I enable it? (If I have the pose prior, then velocity is not needed?) Update: So my understanding is: I have an odometry source, which is a prior for my state variable, either from integrating velocity or lidar odometry poses. And everytime I see an apriltag, which is the observation. Then the factor graph optimize all the states based on all the priors and observations. Is that correct?

[Humhu]

Ideally you should use both if they are enabled. You may want to add a flag to enable/disable the velocity factor, as you did for the prior.