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

+#include <language/utils/OFStream.hpp>
+
+#include <utils/Messenger.hpp>
+
+OFStream::OFStream(const std::string& filename)
+{
+  if (parallel::rank() == 0) {
+    m_fstream.open(filename);
+    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());
+    }
+  }
+}