Feature request: set gravity as origin

[shtern]

Hello,
I've noticed that even with enbabled imu VSLAM is taking the first incoming frame position as origin. I'd expect it to have option to reset the origin position according to gravity data from IMU.
Here is an example of the same functionality in ZED SDK:
set_gravity_as_origin: true # If 'true' align the positional tracking world to imu gravity measurement. Keep the yaw from the user initial pose.