Document the necessary data schema for the Sub9 DVL

[cbrxyz]

Before creating the C++ driver, we will need to expose and uncover the existing structure of the DVL packets. This will allow us to write a custom C++ driver that conforms to these specs.

We will need to know what data is coming in from the DVL, and what packets we can send it in order to interface with the driver (running tests/diagnostics, startup/shutdown, etc.).

This information can be found from the Python driver (by tearing it apart) or by reading the DVL's documentation.