feat(const): updated const to be truly immutable

src/const/public/const.h

@@ -11,11 +11,22 @@
  * @brief Structure to hold a constant's details.
  */
 struct constant {
-  std::string name;         ///< Name of the constant
-  double value;             ///< Value of the constant
-  double uncertainty;       ///< Uncertainty in the constant's value
-  std::string unit;         ///< Unit of the constant
-  std::string reference;    ///< Reference for the constant's value
+  const std::string name;         ///< Name of the constant
+  const double value;             ///< Value of the constant
+  const double uncertainty;       ///< Uncertainty in the constant's value
+  const std::string unit;         ///< Unit of the constant
+  const std::string reference;    ///< Reference for the constant's value
+
+  /**
+   * @brief Parameterized constructor.
+   * @param name The name of the constant.
+   * @param value The value of the constant.
+   * @param uncertainty The uncertainty in the constant's value.
+   * @param unit The unit of the constant.
+   * @param reference The reference for the constant's value.
+   */
+  constant(const std::string& name, double value, double uncertainty, const std::string& unit, const std::string& reference)
+    : name(name), value(value), uncertainty(uncertainty), unit(unit), reference(reference) {}
 
   /**
    * @brief overload the << operator for pretty printing
@@ -81,7 +92,7 @@ public:
    * @param key The name of the constant to retrieve.
    * @return The constant associated with the given key.
    */
-  constant get(const std::string& key);
+  constant get(const std::string& key) const;
 
   /**
    * @brief Overloaded subscript operator to access constants by key.