Refactor execute()

Currently, execute() calls runSimulator() to run the simulation. When
the Python step_init() is implemented (see a later commit), it will
instead call an executeStepInit() that will need to do the same
initialization as in execute() except that it should call a
runSimulatorInit() instead of runSimulator(). In order to avoid code
duplication for execute() and executeStepInit(), execute() is here
refactored into an execute_() method.

opm/simulators/flow/FlowMainEbos.hpp

@@ -245,45 +245,11 @@ namespace Opm
         /// input.
         int execute(int argc, char** argv, bool output_cout, bool output_to_files)
         {
-            try {
+            return execute_(argc, argv, output_cout, output_to_files,
-                // deal with some administrative boilerplate
+                &FlowMainEbos::runSimulator);
-
-                int status = setupParameters_(argc, argv);
-                if (status)
-                    return status;
-
-                setupParallelism();
-                setupEbosSimulator(output_cout);
-                runDiagnostics(output_cout);
-                createSimulator();
-
-                // do the actual work
-                runSimulator(output_cout);
-
-                // clean up
-                mergeParallelLogFiles(output_to_files);
-
-                return EXIT_SUCCESS;
-            }
-            catch (const std::exception& e) {
-                std::ostringstream message;
-                message  << "Program threw an exception: " << e.what();
-
-                if (output_cout) {
-                    // in some cases exceptions are thrown before the logging system is set
-                    // up.
-                    if (OpmLog::hasBackend("STREAMLOG")) {
-                        OpmLog::error(message.str());
-                    }
-                    else {
-                        std::cout << message.str() << "\n";
-                    }
-                }
-
-                return EXIT_FAILURE;
-            }
         }
 
+
         // Print an ASCII-art header to the PRT and DEBUG files.
         // \return Whether unkown keywords were seen during parsing.
         static void printPRTHeader(bool output_cout)
@@ -328,6 +294,49 @@ namespace Opm
           }
         }
 
+    private:
+        // called by execute() or executeInitStep()
+        int execute_(int argc, char** argv, bool output_cout, bool output_to_files,
+            void (FlowMainEbos::* runOrInitFunc)(bool))
+        {
+            try {
+                // deal with some administrative boilerplate
+
+                int status = setupParameters_(argc, argv);
+                if (status)
+                    return status;
+
+                setupParallelism();
+                setupEbosSimulator(output_cout);
+                runDiagnostics(output_cout);
+                createSimulator();
+
+                // if run, do the actual work, else just initialize
+                (this->*runOrInitFunc)(output_cout);
+
+                // clean up
+                mergeParallelLogFiles(output_to_files);
+
+                return EXIT_SUCCESS;
+            }
+            catch (const std::exception& e) {
+                std::ostringstream message;
+                message  << "Program threw an exception: " << e.what();
+
+                if (output_cout) {
+                    // in some cases exceptions are thrown before the logging system is set
+                    // up.
+                    if (OpmLog::hasBackend("STREAMLOG")) {
+                        OpmLog::error(message.str());
+                    }
+                    else {
+                        std::cout << message.str() << "\n";
+                    }
+                }
+
+                return EXIT_FAILURE;
+            }
+        }
     protected:
         void setupParallelism()
         {