XNAT/Core: Fix warnings

This commit fixes the following warnings:

/path/to/CTK/Libs/XNAT/Core/ctkXnatDataModel.cpp:111:52: warning: unused parameter ‘filename’ [-Wunused-parameter]
 void ctkXnatDataModel::downloadImpl(const QString& filename)
                                                    ^

/path/to/CTK/Libs/XNAT/Core/ctkXnatExperiment.cpp:230:53: warning: unused parameter ‘filename’ [-Wunused-parameter]
 void ctkXnatExperiment::downloadImpl(const QString& filename)
                                                     ^


/path/to/CTK/Libs/XNAT/Core/ctkXnatObject.cpp: In member function ‘virtual ctkXnatResource* ctkXnatObject::addResourceFolder(QString, QString, QString, QString)’:
/path/to/CTK/Libs/XNAT/Core/ctkXnatObject.cpp:337:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (unsigned int i = 0; i < children.size(); ++i)
                              ^

/path/to/CTK/Libs/XNAT/Core/ctkXnatProject.cpp:184:50: warning: unused parameter ‘filename’ [-Wunused-parameter]
 void ctkXnatProject::downloadImpl(const QString& filename)
                                                  ^

Libs/XNAT/Core/ctkXnatDataModel.cpp

@@ -110,5 +110,6 @@ void ctkXnatDataModel::fetchImpl()
 //----------------------------------------------------------------------------
 void ctkXnatDataModel::downloadImpl(const QString& filename)
 {
+  Q_UNUSED(filename);
   qDebug() << "ctkXnatDataModel::downloadImpl(const QString& filename) not yet implemented or not available by REST API";
 }

Libs/XNAT/Core/ctkXnatExperiment.cpp

@@ -229,5 +229,6 @@ void ctkXnatExperiment::fetchImpl()
 //----------------------------------------------------------------------------
 void ctkXnatExperiment::downloadImpl(const QString& filename)
 {
+  Q_UNUSED(filename);
   qDebug() << "ctkXnatExperiment::downloadImpl(const QString& filename) not yet implemented or not available by REST API";
 }

Libs/XNAT/Core/ctkXnatObject.cpp

@@ -334,7 +334,7 @@ ctkXnatResource* ctkXnatObject::addResourceFolder(QString foldername, QString fo
 
   ctkXnatResourceFolder* resFolder = 0;
   QList<ctkXnatObject*> children = this->children();
-  for (unsigned int i = 0; i < children.size(); ++i)
+  for (int i = 0; i < children.size(); ++i)
   {
     resFolder = dynamic_cast<ctkXnatResourceFolder*>(children.at(i));
     if (resFolder)

Libs/XNAT/Core/ctkXnatProject.cpp

@@ -183,5 +183,6 @@ void ctkXnatProject::fetchImpl()
 //----------------------------------------------------------------------------
 void ctkXnatProject::downloadImpl(const QString& filename)
 {
+  Q_UNUSED(filename);
   qDebug() << "ctkXnatProject::downloadImpl(const QString& filename) not yet implemented or not available by REST API";
 }