Add Gazebo model(s)

[clydemcqueen]

We could provide Gazebo models for Lutris and Nereo for researchers. These would have the exact frames & measurements in code, and could be used for simulations.

Starting points:

Early model, Gazebo Classic: https://github.com/patrickelectric/bluerov_ros_playground

Gazebo Garden: https://github.com/clydemcqueen/bluerov2_ignition

Possibly newer Gazebo models: https://github.com/Robotic-Decision-Making-Lab/blue

Related to #8

[clydemcqueen]

I'm going to take a crack at an URDF file, mostly to make it easy to publish all of the static transforms.