Call to g_caster function have been replaced by call to
    SignalCaster::getInstance.

    * include/dynamic-graph/signal-caster.h,
    * src/signal/signal-caster.cpp.

     * include/dynamic-graph/signal-caster.h,
     * src/signal/signal-caster.cpp: if a typename is registered several times,
     throw only if pointers to type_info differ. When loading python modules,
     for some reason, global variables are constructed several times.

   * include/dynamic-graph/signal-caster.h,
   * include/dynamic-graph/signal.t.cpp.

     * include/dynamic-graph/exception-abstract.h,
     * include/dynamic-graph/exception-factory.h,
     * include/dynamic-graph/exception-signal.h,
     * include/dynamic-graph/exception-traces.h,
     * include/dynamic-graph/signal-caster.h: This enables uses to catch
       a broader class of exceptions with messages using what() method.

Added boost::ublas::vector example to test_signalcast.

Added unit test of the signal casting facility. Not exporting the SignalCastRegister class anymore (inlined in the .h).

* New class SignalCaster
* Re-added the get, set and trace commands to signals