feat: Updated build tooling for TIGRE

[JeppeKlitgaard]

Adds a maintainable and modern build setup for publishable TIGRE conda packages.

Supercedes: #15
Fixes: #8
Fixes: #5

• Supports TIGRE 3.1 (required for numpy 2)
• Supports Python 3.10 - 3.14

Switches to Pixi-based build backend to reduce maintenance burden.

This allows us to extensively leverage the build setup maintained by TIGRE, even for Conda builds.

Currently this targets a recent revision off of master, which has an outdated version given in pyproject.toml (3.1.0 as opposed to the expected 3.1.1).

I will ask Ander Biguri to cut a release at his convenience, which we can then target. It will likely be 3.1.2.

Note: Currently it is difficult to build single variants using Pixi, so for now this uses a CI matrix only for the operating system, not for the Python version. The Python version matrix is maintained in pyproject.toml as build variants. This is a bit slower since the python versions are then built serially instead of in parallel.

Missing

Currently this just uploads the conda packages as artefacts. It would require a core developer, (@casperdcl?), to do the final plumbing into the ccpi and tomography conda channels.

[JeppeKlitgaard]

Plumbing for uploading to conda channels can be added in this PR or worked on after merging this.

Likely we should hold off on shipping the first build until the next release (3.1.2 or 3.2.0) is cut either way, which I expect to be today or next week