Make NaNHelper constructor explicit

src/utils/NaNHelper.hpp

 #ifndef NAN_HELPER_HPP
 #define NAN_HELPER_HPP
 
+#include <utils/PugsMacros.hpp>
+
 #include <cmath>
 #include <iostream>
 #include <type_traits>
@@ -12,6 +14,7 @@ class NaNHelper
   const T& m_t;
 
  public:
+  PUGS_INLINE
   friend std::ostream&
   operator<<(std::ostream& os, const NaNHelper<T>& helper)
   {
@@ -26,7 +29,9 @@ class NaNHelper
     }
     return os;
   }
-  NaNHelper(const T& t) : m_t{t} {}
+
+  PUGS_INLINE
+  explicit NaNHelper(const T& t) : m_t{t} {}
 
   NaNHelper(NaNHelper&&)      = delete;
   NaNHelper(const NaNHelper&) = delete;