Forward projector

About

This MR adds a Cuda implementation of forward projector (Radon transform). Closes #215 (closed)

It would be good to add an astra backend for completeness, but this would be even better through the corrct package.

To do

• Implement projector
• Unit tests
  • Basic projection
  • Test against astra projector with varying CoR
  • Test some iterative algorithms

Notes

After profiling of iterative reconstruction with EM:

• Nabu's individual Cuda kernels (forward projector, BP) are approx. twice faster than astra's
• Astra is still overall faster for EM_CUDA since the algorithm is natively implemented in C++/Cuda, while Nabu wastes some time in python loops (the effect is smaller for big images)

Using the option "centered_axis": True in backprojector is crucial when center of rotation is off/incorrect.