Make canonical coordinates polymorphic like magnetic_field_t

[krystophny]

Priority: Low

Problem

The canonical coordinate implementations (Boozer, Meiss, Albert, Flux) do not use the polymorphic pattern established by magnetic_field_t. Instead, they use separate modules with similar but not identical interfaces.

Current structure:

• field_can_boozer.f90 - standalone module
• field_can_meiss.f90 - standalone module
• field_can_albert.f90 - standalone module
• field_can_flux.f90 - standalone module

Proposed Solution

Create a canonical_field_t base class:

type, abstract, extends(magnetic_field_t) :: canonical_field_t
contains
    procedure(eval_can_iface), deferred :: evaluate_canonical
    procedure :: evaluate => evaluate_via_canonical
end type

type, extends(canonical_field_t) :: boozer_field_t
type, extends(canonical_field_t) :: meiss_field_t
type, extends(canonical_field_t) :: albert_field_t

Benefits

• Consistent interface across all canonical coordinate types
• Easier to add new canonical coordinate systems
• Better code organization

Caveats

• Significant refactoring effort
• Current approach is well-tested and functional
• Performance impact needs evaluation

Related

• See DOC/coordinates-and-fields.md Section 6