Add ofstream support within the language

On the way rewrite OStream handling: << and >> are now treated as standard binary operators.

This change required the introduction of "global" variable created by module loading since now: cout, cerr and clog are no more keywords but variables of type ostream.

src/language/utils/OFStream.cpp

@@ -6,7 +6,12 @@ OFStream::OFStream(const std::string& filename)
 {
   if (parallel::rank() == 0) {
     m_fstream.open(filename);
-    Assert(m_ostream == nullptr, "ostream was already defined");
-    m_ostream = &m_fstream;
+    if (m_fstream.is_open()) {
+      m_ostream = &m_fstream;
+    } else {
+      std::stringstream error_msg;
+      error_msg << "cannot create file " << rang::fgB::yellow << filename << rang::style::reset;
+      throw NormalError(error_msg.str());
+    }
   }
 }