Switch to ASTRA direct_FP3D/BP3D interface

[wjp]

Description

Switch the Astra wrapper to use Astra's direct_FP3D and direct_BP3D functions for 3D Astra geometries. These functions provide a more concise interface and avoid unnecessary data copies.

This interface is supported from Astra 2.0. Starting from Astra 2.3, these functions also transparently accept DLPack tensors (including torch cpu/cuda tensors).

Example Usage

All unit tests pass for me. This is a result from the CIL PyTorch integration hackathon, and has therefore only seen limited further testing.