introduce SimShowers

Add SimShower header files. Since SimShowers are the same object as
SimClusters, the file only contains typedefs.

interface/SimShower.h

+#ifndef SimShower_h
+#define SimShower_h 
+
+#include "SimCluster.h"
+
+typedef SimCluster SimShower;
+typedef std::vector<SimShower> SimShowerCollection;
+
+#endif
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