Consolidate SIMPLE reference_coordinates with libneo coordinate_system_t

[krystophny]

Priority: Low

Problem

There is functional overlap between SIMPLE's reference_coordinates module and libneo's coordinate_system_t:

SIMPLE reference_coordinates:

• ref_coords global instance
• evaluate_cart(), evaluate_cyl() methods
• VMEC-specific implementation

libneo coordinate_system_t:

• Abstract base with multiple implementations
• evaluate_cart(), evaluate_cyl(), evaluate_metric() methods
• VMEC and chartmap implementations

Both provide coordinate transformations but with different APIs and capabilities.

Proposed Solution

Consider consolidating to use libneo's coordinate_system_t throughout SIMPLE:

1. Replace reference_coordinates usage with coordinate_system_t
2. Or make reference_coordinates a thin wrapper around coordinate_system_t
3. Ensure all needed functionality (Jacobians, metrics) is accessible

Benefits

• Single coordinate transformation API
• Reduced code duplication
• Leverage libneo's more complete implementation

Caveats

• API changes required throughout SIMPLE
• Need to verify libneo provides all required functionality
• May require libneo modifications

Related

• See DOC/coordinates-and-fields.md Section 7