Add differentiation wrt DRR parameters

[eigenvivek]

Want the gradients wrt to

• Camera.center (the 3D coordinates of the camera)
• Detector.center (the 3D coordinates of the center of the detector plane)
• Detector.normal (the orientation of the detector plane)

[eigenvivek]

Automatic differentiability w/ Zygote doesn't work out of the box for these parameters... probably need to implement a few custom gradient rules with ChainRulesCore, but I'm very unsure where the underlying issue is

[eigenvivek]

Also validate the custom rules with finite difference

[eigenvivek]

(and benchmark runtime against finite differences)