Code cleaning after rewriting (marginal) of sig-cast. Introduction of a specific tracer behaviour for matrix and vector.

Corrected a segfault problem in the test pool. The problem at the global level remains. At least, the test passes now.

  This reverts commit 4513ebe9.

      Remove pool::getSignal().

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

       This reverts commit 0b9f9528.
       Move python interpreter in dynamic-graph-python.

     * src/CMakeLists.txt,
     * tools/CMakeLists.txt.

    * 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.