Optimize C++ Faust*Dense-matrix especially for pyfaust.

Removing many useless copies. For instance, the new multiply function works directly in numpy allocated memory instead of copying back and forth from/to Faust::MatDense.
In that purpose it uses Eigen Map (views).

 hinria  …  build  wrapper  python  python3 ./test_mul_matdense.py
time F@M: 72.12258507497609
 hinria  …  build  wrapper  python  python3 ./test_mul_matdense.py
time F@M: 55.56243183999322

Taking into account that there is several methods implemented to multiply a Faust by a vector or by a matrix. For now, this optimization is available only for the FaustMulMode::DEFAULT method (see issue #188).