Allow selecting a faster second-order static GS operator

[nicamo]

Summary

This adds a small, opt-in choice of finite-difference order for the static Grad-Shafranov solver.

• Add gs_operator_order={2, 4} to GSstaticsolver.NKGSsolver.
• Keep fourth order as the default, preserving existing behaviour.
• Select FreeGS4E's existing GSsparse second-order operator or GSsparse4thOrder fourth-order operator.
• Reject unsupported values with a clear ValueError.
• Document the option and accuracy trade-off in Example 2.
• Add tests for the second-order equilibrium and invalid operator orders.

No stencil or numerical solver implementation is duplicated in FreeGSNKE.

Linear-solver path

Although the FreeGS4E factory is named createVcycle, FreeGSNKE configures it with:

nlevels=1
direct=True

This constructs MGDirect, performs a sparse LU factorisation with SciPy, and uses direct back-solves. No multigrid restriction, prolongation, smoothing, or V-cycle is performed.

The second-order benefit therefore comes primarily from constructing and factorising a sparser matrix.

Performance

On the standard 65 x 129 static-solver test equilibrium:

Measurement	Fourth order	Second order	Change
Solver construction	0.274 s	0.127 s	54% faster
Nonlinear solve after construction	0.205 s	0.208 s	essentially unchanged
Total	0.487 s	0.351 s	28% faster

The total gain will be largest when solver objects are constructed frequently. Reusing an already-factorised solver reduces the benefit because repeated direct back-solves cost approximately the same.

Accuracy and compatibility

Second order has the expected spatial discretisation accuracy trade-off and is therefore opt-in. The default remains gs_operator_order=4.

The regression test verifies that the second-order equilibrium agrees with the established static equilibrium baseline within 0.3% of the reference flux range.

Verification

• python -m pytest -q freegsnke/tests/test_static_solver.py
• 3 passed
• Example 2 remains valid notebook JSON

[kpentland]

Nice addition if users need speed!