Mass: make the constructor explicit.

Implicit initialisation from a std::string could be dangerous, so, let's
avoid that.

src/Mass/Mass.h

@@ -24,7 +24,7 @@ enum class MassState : std::uint8_t {
 
 class Mass {
     public:
-        Mass(const std::string& filename);
+        explicit Mass(const std::string& filename);
 
         Mass(const Mass&) = delete;
         Mass& operator=(const Mass&) = delete;