Make a singleton out of the PythonCaller

Prevents unnecessary instances of PythonCaller.

DRAMSys/traceAnalyzer/tracefiletab.cpp

@@ -93,8 +93,7 @@ void TraceFileTab::exportAsVCD()
     QString filename = QFileDialog::getSaveFileName(this, "Export to VCD", "",
                                                     "VCD files (*.vcd)");
 
-    PythonCaller pythonCaller;
-    QString dump = pythonCaller.exportAsVcd(path);
+    QString dump = PythonCaller::instance().exportAsVcd(path);
 
     if (filename != "") {
         QFile file(filename);