add simulators

[bluesquall]

Add basic simulators for the radiometer and the vehicle.

An old draft for the radiometer is already in this repository, but it may not work anymore.

A basic simulator of the vehicle could implement the isolum-following control loop with low-fidelity vehicle dynamics, publish simulated depth, and that could feed back through a time/depth environmental filter, into the simulated radiometer, to close the entire loop.

With all these parts in place, any one piece could be replaced with an actual physical component for integrated regression testing.

[bluesquall]

Start with just publishing the simulated radiometer data on RAD1u.