Support unpreconditioned GMRES

Summary

Add support for unpreconditioned GMRES when precond = "none".

Rationale

Unpreconditioned GMRES provides a useful baseline for evaluating new preconditioners, including improvements in iteration count and total runtime with setup costs included. It also helps with debugging by narrowing down the source of failures through preconditioned and unpreconditioned runs.

Description

A minimal implementation would be to add PreconditionerIdentity, a class derived from Preconditioner that represents $M = I$ and is therefore mathematically equivalent to applying no preconditioner. This approach would simply reuse the existing preconditioner interface and GMRES implementation with minimal changes.

Alternatively, GMRES could provide a dedicated unpreconditioned path that does not use $M$ at all, but this would require more changes.

Additional information

No response

Locate the existing Preconditioner interface, GMRES implementation, and handling of precond = "none"; then inspect how current preconditioners are selected and applied. Implement support using the existing interface as described, and verify that unpreconditioned GMRES runs successfully and can provide a baseline for preconditioned runs.