Fix incorrect timing values

include/openmc/timer.h

@@ -31,7 +31,7 @@ public:
 private:
   bool running_ {false}; //!< is timer running?
   std::chrono::time_point<clock> start_; //!< starting point for clock
-  double elapsed_; //!< elasped time in [s]
+  double elapsed_ {0.0}; //!< elasped time in [s]
 };
 
 //==============================================================================

src/timer.cpp

@@ -28,7 +28,7 @@ double Timer::elapsed()
 {
   if (running_) {
     std::chrono::duration<double> diff = clock::now() - start_;
-    return elapsed_ += diff.count();
+    return elapsed_ + diff.count();
   } else {
     return elapsed_;
   }