Bouncy depth hold, odd IMU behavior?

[clydemcqueen]

I am seeing some "bouncy" depth hold behavior using SITL+Gazebo that I'm not seeing in the default SITL. It may be caused by the IMU simulation. With Release Sub-4.1 branch, release 4.1.0.

SITL without Gazebo, holding depth, about 30 seconds:

SITL with Gazebo, holding depth, about 30 seconds -- notice the IMU:

[clydemcqueen]

The default SITL barometer is noisy -- which makes sense for plane and copter, but not for sub. Quick fix:

param set SIM_BARO_RND 0.01