feat: create base_field and add a field directory

include/samurai/crtp.hpp

@@ -0,0 +1,48 @@
+// Copyright 2018-2025 the samurai's authors
+// SPDX-License-Identifier:  BSD-3-Clause
+
+#pragma once
+
+namespace samurai
+{
+    template <class D>
+    class crtp_base
+    {
+      public:
+
+        using derived_type = D;
+
+        const derived_type& derived_cast() const& noexcept;
+        derived_type& derived_cast() & noexcept;
+        derived_type derived_cast() && noexcept;
+
+      protected:
+
+        crtp_base()  = default;
+        ~crtp_base() = default;
+
+        crtp_base(const crtp_base&)            = default;
+        crtp_base& operator=(const crtp_base&) = default;
+
+        crtp_base(crtp_base&&) noexcept            = default;
+        crtp_base& operator=(crtp_base&&) noexcept = default;
+    };
+
+    template <class D>
+    inline auto crtp_base<D>::derived_cast() const& noexcept -> const derived_type&
+    {
+        return *static_cast<const derived_type*>(this);
+    }
+
+    template <class D>
+    inline auto crtp_base<D>::derived_cast() & noexcept -> derived_type&
+    {
+        return *static_cast<derived_type*>(this);
+    }
+
+    template <class D>
+    inline auto crtp_base<D>::derived_cast() && noexcept -> derived_type
+    {
+        return *static_cast<derived_type*>(this);
+    }
+}