COMP: correct timer clock type

Trigger compilation error on macOSx

Base/Core/imstkTimer.h

@@ -112,7 +112,7 @@ private:
     TimerState state;
     std::vector<double> lapTimes;
     std::vector<std::string> lapNames;
-    std::chrono::system_clock::time_point wallClockTimeKeeper; ///> time keeper for wall clock time
+    std::chrono::steady_clock::time_point wallClockTimeKeeper; ///> time keeper for wall clock time
 };
 
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