[kernel] provide initial value to Simulation::_T, fixes test_serialization4
[kernel] provide initial value to Simulation::_T, fixes test_serialization4

issue was that a random value was written to XML for _T, and would
throw an exception on loading because it was legitimately not a number
representable by double.

fperignon/siconos,radarsat1/siconos,fperignon/siconos,radarsat1/siconos,siconos/siconos,siconos/siconos,siconos/siconos,fperignon/siconos,siconos/siconos,radarsat1/siconos,fperignon/siconos,radarsat1/siconos,radarsat1/siconos,fperignon/siconos