refactor(const): header guard -> pragma once

also removed unused includes

src/const/public/const.h

@@ -18,13 +18,10 @@
 //   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 //
 // *********************************************************************** */
-#ifndef CONST_H
+#pragma once
-#define CONST_H
 
 #include <string>
-#include <fstream>
 #include <iostream>
-#include <vector>
 #include <set>
 #include <map>
 
@@ -96,7 +93,7 @@ private:
 public:
 
   /**
-   * @brief get instance of constants singelton
+   * @brief get instance of constants singleton
    * @return instance of constants
    */
   static Constants& getInstance() {
@@ -108,7 +105,7 @@ public:
    * @brief Check if constants are loaded.
    * @return True if constants are loaded, false otherwise.
    */
-  bool isLoaded() { return loaded_; }
+  bool isLoaded() const { return loaded_; }
 
   /**
    * @brief Get a constant by key.
@@ -139,5 +136,3 @@ public:
   std::set<std::string> keys() const;
 
 };
-
-#endif