Add tethys model with DVL sensor

[iche033]

🎉 New feature

Summary

Adds tethys underwater vehicle that contains various sensors. One of them is a DVL sensor which is a new harmonic feature.

The dvl sensor uses bottom tracking model. It's placed close to the surface of the water because the sea bed is currently quite shallow.

Test it

Requires this sensors system deadlock fix: gazebosim/gz-sim#2141

export GZ_SIM_RESOURCE_PATH=`pwd`/harmonic_demo:$GZ_SIM_RESOURCE_PATH
gz sim -v 4 -r harmonic_demo/harmonic.sdf

# move the vehicle
gz topic -t /model/tethys/joint/propeller_joint/cmd_thrust -m gz.msgs.Double -p 'data: -31'

# echo the dvl velocity topic and make sure that the velocities are changing
gz topic -e -t /tethys/dvl/velocity

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[iche033]

merging this as the demo was already presented