Fix Python API example files

[rajat2004]

Changes -

1. Fix hello_car.py print, temp folder for images, cleanup
2. Delete legacy_hello_car.py
3. Add utility functions to check for NaN in Pose (simGetObjectPose() sometimes returns NaN #2695)
4. Fix cv_mode.py
5. [Unity] Don't send Alpha channel in image
6. [Utils] Use cv2 to write images (Difference in image when using sim.getImages to retrieve a compressed and uncompressed image. #2892)

Just some things noticed when trying to figure out why Unity Car APIs aren't working which has been reported on multiple issues. I'm not seeing anything wrong or different from the Unreal implementation right now. Ping @madratman @saihv

Unity returns RGBA while Unreal does RGB (Not after this PR)

[rajat2004]

Actually client.reset() does work in CV mode

[madratman]

My guess would be we can change the buffer type of Unity from RGBA to RGB by not copying over the alpha channel?
It would be the equivalent of https://github.com/microsoft/AirSim/pull/1910/files#diff-598dac89bb5af3518db2ff6858f0a389L118. Can you try finding these lines in Unity?

[rajat2004]

@madratman I think this is the one we need - https://github.com/microsoft/AirSim/blob/master/Unity/UnityDemo/Assets/AirSimAssets/Scripts/Utilities/DataCaptureScript.cs#L153
I could comment it out, will backwards compatibility be a problem. since no. of channels will change? I'll still test it out and commit the change to this PR

[madratman]

No, I think it would be for the better as it establishes consistency in num of channels in UE and Unity,

[rajat2004]

Made the Unity change, and reversed the channel change in the script.
2 other things I noted from testing -

1. Unity API (atleast Car movement one, need to check others also) is broken, Car doesn't move (Related issues Car in UnityDemo Project does not move using PythonClient scripts #2631, how to use python API to control car in unity? #2636)
2. Uncompressed images are corrupted I think, they don't get saved, and cv2.imshow shows a blank screen. I got to know about this 1-2 days back when doing some work with Recording where setting Compress to False didn't save the images
   Edit: Nothing, just forgot waitKey while doing imshow

Forgot that I had mentioned the 1st problem in the PR description itself...

[madratman]

Merging this.. let's track the remaining issues in new PRs

[rajat2004]

#2937 fixes the Unity Car API problem, still needs a bit more work though