Added support for BlueROV2 Heavy

[evan-palmer]

Changes Made

This PR introduces the BlueROV2 Heavy model. Furthermore, a few small bugs were squashed, support for adding new models was improved, and tried to improve compliance with conventions established in the SITL_Models project to make it easier to merge new models over there.

I remembered to squash my commits this time around too :wink:

Associated Issues

• Closes #8

Files Changed

• README.md: The README was updated to document usage for the new model.
• models/bluerov2/**: Modified some file names and added documentation to match the conventions of the SITL_Models project, fixed a bug from the previous commit which used incorrect propeller meshes for some thrusters (didn't match the thruster configuration specified here), and improved the alignment of the thrusters that were previously unaligned.
• models/bluerov2_heavy/**: Introduced the BlueROV2 Heavy model which uses hydrodynamic parameters determined experiementally from previous work.
• scripts/*.sh: Updated the bash scripts to support different models
• generate_model.py: I had to do a fairly large re-write of this script to support generating different types of models. To make the tuning process a bit easier, I also added support for loading configurations from a YAML configuration file. I'd like to note that I considered placing all of the parameters into a dict directly instead of going from class -> dict, but I thought that a simple class might make it easier to add additional functionality in the future.
• worlds/**: Added a new world file for the heavy.

[evan-palmer]

Wow, this is amazing! What a great contribution. Sorry it took so long to review this. A few nits.

Thank you for the feedback! @ssicari was kind enough to make the changes that you have requested.

[clydemcqueen]

Fantastic stuff. There are a few more changes I'd like to see, but I'm getting a bit tangled up in git, so I'm going to push them as a separate PR. Thanks again @evan-palmer and @ssicari !