Modernize magfie interface to eliminate global procedure pointer

[krystophny]

Priority: Low

Problem

The magfie interface uses a module-level procedure pointer (magfie_ptr) to select field evaluation at runtime. This global state pattern:

procedure(magfie_iface), pointer :: magfie_ptr => null()

subroutine init_magfie(mode)
    select case (mode)
        case (VMEC)
            magfie_ptr => magfie_vmec
        case (BOOZER)
            magfie_ptr => magfie_can_boozer
        ! ...
    end select
end subroutine

While functional, this could be modernized to use object-oriented dispatch.

Proposed Solution

Consider passing a field object directly to integrators instead of using global procedure pointer:

type(orbit_symplectic_t) :: orbit
class(magnetic_field_t), pointer :: field

call orbit%init(field)  ! Field object passed directly
call orbit%timestep(z, dtau)  ! Uses field internally

Benefits

• Eliminates global state
• Enables multiple simultaneous field configurations
• Better encapsulation

Caveats

• Significant refactoring effort
• Current approach works fine for typical use cases
• May impact performance if virtual dispatch adds overhead in hot loops

Related

• See DOC/coordinates-and-fields.md Section 4