Added Black Hole Raytracing Example

[edwinsolisf]

Added a black hole raytracing example.

Description

Added a Black Hole Raytracing Example to the ArrayFire example list. It was added to the pde folder and it contains 1 image that was added to the assets/examples/images repo folder. It contains three scenes to display.

• Is this a new feature or a bug fix?: Neither
• Potential impact on specific hardware, software or backends: None
• New functions and their functionality: None
• Can this PR be backported to older versions?: Yes
• Future changes not implemented in this PR: None

Changes to Users

• New example project in the arrayfire example list
• No action required by the user

Checklist

• [x] Rebased on latest master
• [x] Code compiles
• [ ] Tests pass
• [x] Functions added to unified API
• [x] Functions documented