This reverts commit 046c29c2.

	  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.

    * CMakeLists.txt,
    * doc/CMakeLists.txt,
    * src/dynamic_graph/signal_base.py.