changes to exceptions that returned pointer to temporary object

Fixed compiler warnings

SpMat.h

@@ -39,10 +39,10 @@ class SpMatException: public std::exception
 private:
   std::string m_msg;
 public:
-  SpMatException(const std::string& msg) throw(): m_msg(msg) {}
+  SpMatException(const std::string& msg) throw(): m_msg(std::string("SpMat::")+msg) {}
 
   virtual const char * what() const throw() {
-    return std::string("SpMat::" + m_msg).c_str();
+    return(m_msg.c_str());
   }
 
   ~SpMatException() throw() {}