feat(dimensions): added second partial type and new dimensions

src/include/serif/dimensions/partial.hpp

@@ -27,6 +27,22 @@ namespace serif::dimensions {
     template <ThermodynamicQuantityType Quantity>
     struct WithRespectTo final {};
 
+    /** @brief
+     * A second EOS partial retains both differentiation quantities in its type.
+     */
+    template <ThermodynamicQuantityType OutputQuantity, ThermodynamicQuantityType FirstInput, ThermodynamicQuantityType SecondInput>
+    class SecondPartialDerivative final {
+    public:
+        /** @brief Store an explicitly evaluated second partial; this type does not differentiate. */
+        explicit constexpr SecondPartialDerivative(double value) noexcept : m_value(value) {}
+
+        /** @brief Return the numeric partial in the caller's physical units. */
+        [[nodiscard]] constexpr double value() const noexcept { return m_value; }
+
+    private:
+        double m_value;
+    };
+
     /**
      * @class PartialDerivative
      *
@@ -188,4 +204,4 @@ namespace serif::dimensions {
     private:
         double m_value; ///< Numeric value associated with the partial. If this is known at compile time then the compiler can optimize operations with partial.value() to be equivalent to compile time double operations.
     };
-}
+}