Make a singleton out of the PythonCaller

Prevents unnecessary instances of PythonCaller.

DRAMSys/traceAnalyzer/traceanalyzer.cpp

@@ -33,6 +33,7 @@
  *    Janik Schlemminger
  *    Robert Gernhardt
  *    Matthias Jung
+ *    Derek Christ
  */
 
 #include "traceanalyzer.h"
@@ -122,8 +123,7 @@ void TraceAnalyzer::openTracefile(const QString &path)
     ui->actionReload_all->setEnabled(true);
     ui->actionSaveChangesToDB->setEnabled(true);
 
-    PythonCaller pythonCaller;
-    if (pythonCaller.vcdExportDependenciesAvailable())
+    if (PythonCaller::instance().vcdExportDependenciesAvailable())
         ui->actionExportAsVCD->setEnabled(true);
 
     ui->actionClose_all->setEnabled(true);