Make the bool conversion operator a const method.

Libs/CommandLineModules/Core/ctkCmdLineModuleReference.cpp

@@ -70,7 +70,7 @@ ctkCmdLineModuleReference &ctkCmdLineModuleReference::operator =(const ctkCmdLin
 }
 
 //----------------------------------------------------------------------------
-ctkCmdLineModuleReference::operator bool()
+ctkCmdLineModuleReference::operator bool() const
 {
   return !d->RawXmlDescription.isEmpty();
 }

Libs/CommandLineModules/Core/ctkCmdLineModuleReference.h

@@ -47,7 +47,7 @@ public:
   ctkCmdLineModuleReference(const ctkCmdLineModuleReference& ref);
   ctkCmdLineModuleReference& operator=(const ctkCmdLineModuleReference& ref);
 
-  operator bool();
+  operator bool() const;
 
   ctkCmdLineModuleDescription description() const;