Unity weather parameters, weather HUD, and a visual effect for snow

[ForrestTrepte]

This is a Unity implementation of weather analogous to AirSim's Unreal weather system. Currently only the snow weather type is implemented.

• Weather class stores global weather settings and applies them to instances of weather effects in the scene.
• AirSimGlobal singleton provides access to global Weather object.
• WeatherFX implements weather visuals surrounding a vehicle.
• HUD contains weather menu for enabling weather and setting snow quantity. Triggered by F10.
• Snow particle system.

This PR is a step towards #2395, but is not yet a complete implementation of weather in Unity. I haven't contributed to AirSim before, so please let me know if there's something that doesn't make sense or isn't following conventions.

[ForrestTrepte]

FYI, this is work from the Microsoft Hackathon 2020.

[ForrestTrepte]

I am unfamiliar with this build system. Can anyone help me understand why the CI build failed on Ubuntu 16.04? Perhaps related to this?

+echo '### Eigen is not installed. Please run setup.sh first.'
### Eigen is not installed. Please run setup.sh first.
+exit 1

[rajat2004]

It's just a rate limit on Travis which happens sometimes

+wget https://github.com/Microsoft/AirSim/releases/download/v1.2.0/car_assets.zip
--2020-08-04 01:31:41--  https://github.com/Microsoft/AirSim/releases/download/v1.2.0/car_assets.zip
Resolving github.com (github.com)... 140.82.113.3
Connecting to github.com (github.com)|140.82.113.3|:443... connected.
HTTP request sent, awaiting response... 429 too many requests
2020-08-04 01:31:41 ERROR 429: too many requests.

You could close and reopen the PR or rebase against master, or if anyone has permissions, they can restart the Travis build

[ForrestTrepte]

Thanks @rajat2004! I made an update and the checks passed this time.

[zimmy87]

Tested this locally and it works for me, however there's a few compile error and warnings we'd like to see addressed before merging this.

[ForrestTrepte]

Thanks for the review, @zimmy87. I made changes that I think are straightforward and resolve these issues. However, I wasn't able to test running them. It has been a long time since I worked on this and, currently, Unity crashes whenever I play the DroneDemo scene in the editor. It doesn't seem to be related to my changes. I crash with the original version of my PR branch, with these updates, or with the latest master branch. I'm not sure what has gone wrong and don't have time to investigate. Would you be willing to test it locally again?

[zimmy87]

Tested this locally again and the latest version is working for me. There's one remaining item that I think is a a typo, once that's addressed, I think this will be ready for merging.

[ForrestTrepte]

Thanks for fixing my typo, testing, and merging @zimmy87!!