Make IMU, GPS, pinger and ball shooter joints fixed

[stepanove]

This is a proposed fix for the issue #866.
I'm new to VRX, but as far as I understand, the ball shooter is not supposed to be actuated. Actually, the current joints configuration is wrong anyway, it has two joints both rotating around Z, the second joint should rotate around Y, I suppose. It's obvious that IMU, GPS and acoustic sensors should be fixed. It looks like all of this is just a kind of a copy-paste mistake.
So my suggestion is to make all the following joints fixed:

• wamv/ball_shooter_base_joint
• wamv/ball_shooter_launcher_joint
• wamv/gps_wamv_joint
• wamv/imu_wamv_joint
• wamv/receiver_pinger_joint