Function rgpot_potential_new 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn rgpot_potential_new(
    callback: PotentialCallback,
    user_data: *mut c_void,
    free_fn: Option<unsafe extern "C" fn(*mut c_void)>,
) -> *mut rgpot_potential_t

Create a new potential handle from a callback function pointer.

• callback: the function that performs the force/energy calculation.
• user_data: opaque pointer forwarded to every callback invocation (typically a pointer to the C++ potential object).
• free_fn: optional destructor for user_data. Pass NULL if the caller manages the lifetime externally.

Returns a heap-allocated rgpot_potential_t*, or NULL on failure. The caller must eventually pass the returned pointer to rgpot_potential_free.