Add support for solver cuPDLPx

[mal84emma]

Changes proposed in this Pull Request

Add support for GPU-accelerated solver cuPDLPx

Note: due to the lower numerical precision of this solver, some of the equality assertions in the tests have been replaced with np.isclose
Also note: this solver does not support QP or MIP, and currently the file IO is not available through the Python API (will add when later versions support it)

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[fneum]

Looks pretty good!

The higher tolerances in this PR largely apply to all solvers. It could be an option to introduce a constant that sets the tolerance based on whether it's a GPU or CPU-based solver.

[mal84emma]

Great point :) I've just added global variables to the tests that set the acceptable relative solution tolerances for CPU and GPU based solvers.