beginning of the map.

ref https://github.com/stack-of-tasks/sot-core/issues/58

Spotted by M. Naveau.

    g_pool and g_factory global variables have been replaced by static methods
    getInstance in each class.
    getInstance returns a pointer to the unique instance of the class and
    creates it if needed.

      Remove pool::getSignal().

       This reverts commit 8fca0b1a.
       Move python interpreter in dynamic-graph-python.

    * include/dynamic-graph/interpreter.h,
    * src/CMakeLists.txt,
    * src/dgraph/interpreter.cpp,
    * tests/CMakeLists.txt,
    * tools/dg-python.cpp: new,
    * tools/CMakeLists.txt: this adds a dependency to  python in dynamic-graph.

	  * include/CMakeLists.txt,
	  * include/dynamic-graph/command.h: new,
	* include/dynamic-graph/entity.h,
	* include/dynamic-graph/parameter.h: new,
	* include/dynamic-graph/value.h: new,
	* src/command/command.cpp: new,
	* src/command/value.cpp: new,
	* src/dgraph/entity.cpp.

    * include/dynamic-graph/factory.h,
    * src/dgraph/factory.cpp: method name is FactoryStorage::listEntities.