refactor: move AbstractIntegrator and AdaptiveIntegrator to diffeq::core namespace

- Move AbstractIntegrator to diffeq::core namespace
- Move AdaptiveIntegrator to diffeq::core namespace
- Update Euler, RK4, and ImprovedEuler integrators to use correct namespace
- Fix inheritance from core::AbstractIntegrator

Author: WenyinWei

include/core/abstract_integrator.hpp

@@ -4,6 +4,8 @@
 #include <type_traits>
 #include "concepts.hpp"
 
+namespace diffeq::core {
+
 // Abstract integrator base class
 template<system_state S>
 class AbstractIntegrator {
@@ -43,3 +45,5 @@ class AbstractIntegrator {
     // Helper function for derived classes to update time
     void advance_time(time_type dt) { current_time_ += dt; }
 };
+
+} // namespace diffeq::core

include/core/adaptive_integrator.hpp

@@ -12,6 +12,8 @@
 #include <core/abstract_integrator.hpp>
 #include <core/state_creator.hpp>
 
+namespace diffeq::core {
+
 // Abstract adaptive integrator with error control
 template<system_state S>
 class AdaptiveIntegrator : public AbstractIntegrator<S> {
@@ -122,3 +124,5 @@ class AdaptiveIntegrator : public AbstractIntegrator<S> {
         return std::max(dt_min_, std::min(dt_max_, current_dt * factor));
     }
 };
+
+} // namespace diffeq::core

include/integrators/ode/euler.hpp

@@ -15,9 +15,9 @@ namespace diffeq {
  * Usage: Educational purposes, simple problems
  */
 template<system_state S>
-class EulerIntegrator : public AbstractIntegrator<S> {
+class EulerIntegrator : public core::AbstractIntegrator<S> {
 public:
-    using base_type = AbstractIntegrator<S>;
+    using base_type = core::AbstractIntegrator<S>;
     using state_type = typename base_type::state_type;
     using time_type = typename base_type::time_type;
     using value_type = typename base_type::value_type;

include/integrators/ode/improved_euler.hpp

@@ -15,9 +15,9 @@ namespace diffeq {
  * Usage: Simple problems where RK4 is overkill
  */
 template<system_state S>
-class ImprovedEulerIntegrator : public AbstractIntegrator<S> {
+class ImprovedEulerIntegrator : public core::AbstractIntegrator<S> {
 public:
-    using base_type = AbstractIntegrator<S>;
+    using base_type = core::AbstractIntegrator<S>;
     using state_type = typename base_type::state_type;
     using time_type = typename base_type::time_type;
     using value_type = typename base_type::value_type;

include/integrators/ode/rk4.hpp

@@ -16,9 +16,9 @@ namespace diffeq {
  * Usage: General-purpose ODE integration
  */
 template<system_state S>
-class RK4Integrator : public AbstractIntegrator<S> {
+class RK4Integrator : public core::AbstractIntegrator<S> {
 public:
-    using base_type = AbstractIntegrator<S>;
+    using base_type = core::AbstractIntegrator<S>;
     using state_type = typename base_type::state_type;
     using time_type = typename base_type::time_type;
     using value_type = typename base_type::value_type;