This function is equivalent to print() under python,
       is equivalent to str() in dg-python.

	  Executable dg-python emulates a minimal python interpreter.
	  The interpreter is able to
	    - execute single statements: (import, print,...),
	    - evaluate expressions.
	  Expressions and statements are separated by semi-colons. Several
	  statements and/or evaluations between two semi-colons yield an
	  undefined behavior.

    C++ entity might still be in use.

    * src/dynamic-graph-py.cc,
    * src/dynamic_graph/signal_base.py,
    * src/signal-base-py.cc.

    * src/signal-caster-py.cc: new.

      * src/signal-base-py.cc.

       * src/dynamic_graph/entity.py.

    * src/dynamic-graph-py.cc,
    * src/dynamic_graph/signal_base.py,
    * src/signal-base-py.cc.

    * src/CMakeLists.txt,
    * src/dynamic-graph-py.cc.

       * src/dynamic-graph-py.cc: dynamic_graph.wrap.error ->
         dynamic_graph.error.

	* src/dynamic-graph-py.cc.

       * src/dynamic_graph/signal_base.py.

      * src/entity-py.cc.

       * src/entity-py.cc.