Add method to interprete a string as a python command in class Interpreter.

    * 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.