RKC: actual_integrator.H and actual_integrator_sdc.H share the same include guard

[zingale]

Summary

Both files use:

#ifndef actual_integrator_H
#define actual_integrator_H

Why this is a bug

If a translation unit includes both headers (directly or transitively), the second include is skipped entirely because the guard macro is already defined. This can silently hide required declarations/definitions and create build or behavior differences depending on include order.

Location

• integration/RKC/actual_integrator.H
• integration/RKC/actual_integrator_sdc.H

Suggested fix

Use unique include guards per file (or #pragma once), e.g. actual_integrator_strang_H and actual_integrator_sdc_H.