Add signals of type std::string.

Author: florent-lamiraux

src/signal/signal-cast-helper.cpp

@@ -82,6 +82,7 @@ DefaultCastRegisterer<bool> bool_reg;
 DefaultCastRegisterer<dynamicgraph::Vector> vectorCastRegisterer;
 DefaultCastRegisterer<dynamicgraph::Matrix> matrixCastRegisterer;
 DefaultCastRegisterer<boost::posix_time::ptime> ptimeCastRegisterer;
+DefaultCastRegisterer <std::string> stringCastRegisterer;
 } // end of anonymous namespace.
 
 } // namespace dynamicgraph