Use OFString directly instead of std::string

Visual Studio complains that it can't convert std::string into OString
for the lines "scu.setPeerHostName(host);" and
"scu.setPeerAETitle(peerAET);". There is no advantage here to use
std::string so let's use OFString directly.

Applications/ctkDICOMDemoSCU/ctkDICOMDemoSCUMain.cpp

@@ -44,9 +44,9 @@ int main(int argc, char** argv)
     return 2;
   } 
   
-  OString host(argv[1]);
+  OFString host(argv[1]);
   unsigned int port = atoi(argv[2]);
-  OString peerAET;
+  OFString peerAET;
   if (argc > 3)
   {
     peerAET = argv[3];