Add 'smooth_tag' for smooth kernels + trait to detect at compile-time if

a kernel is smooth.

include/scalfmm/matrix_kernels/gaussian.hpp

@@ -34,6 +34,8 @@ namespace scalfmm::matrix_kernels
         static constexpr std::size_t km{1};   // The number of inputs of the kernel.
         static constexpr std::size_t kn{1};   // The number of outputs of the kernel.
 
+        static constexpr bool is_smooth{true};   // Specify that the kernel is smooth.
+
         // Mandatory type
         template<typename ValueType>
         using matrix_type = std::array<ValueType, kn * km>;   // Matrix type that is used in the kernel.

include/scalfmm/meta/traits.hpp

@@ -6,7 +6,7 @@
 // --------------------------------
 // See LICENCE file at project root
 // File : scalfmm/meta/traits.hpp
-// --------------------------------
+// -------------------------------
 #ifndef SCALFMM_META_TRAITS_HPP
 #define SCALFMM_META_TRAITS_HPP
 
@@ -920,6 +920,34 @@ namespace scalfmm::meta
     template<typename T>
     inline static constexpr bool is_particle_v = is_particle<T>::value;
 
+    /**
+     * @brief
+     *
+     * @tparam T
+     */
+    template<typename MatrixKernelType, typename = void>
+    struct is_smooth : std::false_type
+    {
+    };
+
+    /**
+     * @brief
+     *
+     * @tparam T
+     */
+    template<typename MatrixKernelType>
+    struct is_smooth<MatrixKernelType, std::void_t<decltype(MatrixKernelType::is_smooth)>> : std::true_type
+    {
+    };
+
+    /**
+     * @brief
+     *
+     * @tparam T
+     */
+    template<typename MatrixKernelType>
+    inline static constexpr bool is_smooth_v = is_smooth<MatrixKernelType>::value;
+
     /**
      * @brief
      *