feat(const.h): added basic interface definition for constants module

src/const/public/const.h

@@ -0,0 +1,59 @@
+#ifndef CONST_H
+#define CONST_H
+
+#include <string>
+#include <fstream>
+#include <iostream>
+#include <vector>
+#include <map>
+
+struct constant {
+  std::string name;
+  double value;
+  double uncertainty;
+  std::string unit;
+  std::string reference;
+}
+
+class constants {
+private:
+  bool loaded_ = false;
+  std::map<std::string, constant> constants_;
+
+  bool load(const std::string& filename);
+
+public:
+  constants();
+
+  constants(const std::string& filename);
+
+  bool is_loaded() { return loaded_; }
+
+  bool initialize(const std::string& filename);
+
+  constant get(const std::string& key);
+
+  constant operator[](const std::string& key) const {
+    auto it = constants_.find(key);
+    if (it != constants_.end()) {
+      return it->second;
+    } else {
+      throw std::out_of_range("Constant '" + key + "' not found.");
+    }
+  };
+  
+  bool has(const std::string& key) const {
+    return constants_.find(key) != constants_.end();
+  };
+
+  std::Vector<std::string> keys() const {
+    std::Vector<std::string> keys;
+    for (auto it = constants_.begin(); it != constants_.end(); ++it) {
+      keys.push_back(it->first);
+    }
+    return keys;
+  };
+
+};
+
+#endif