feat(const): added << operator to constant so that constants

also added tests for the stream operator

src/const/public/const.h

@@ -16,6 +16,16 @@ struct 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
+
+  /**
+   * @brief overload the << operator for pretty printing
+   */
+   friend std::ostream& operator<<(std::ostream& os, const constant& c) {
+     os << "<" << c.name << ": ";
+     os << c.value << "±" << c.uncertainty << " ";
+     os << c.unit << " (" << c.reference << ")>\n";
+     return os;
+   }
 };
 
 /**