Update many things mostly around Faust scalar types conversions + wrapper building.

- New tests for pyfaust.Faust.astype,
- New C++ MatDense/MatSparse::cast() on CPU and GPU.
- TransformHelper::real() is replaced by cast() for CPU and GPU.
- Fix of MatSparse<GPU2>::to_real
- Complete pyfaust.astype (conversion to float32 handled).
- Refactoring of scalar types conversion with (new) to_double/to_float functions (replacing real function) in python wrappers and (already existing) faust_single/faust_double in matlab wrappers.
- Adjust parallel compilation of python wrapper (started in 7e2c881b).
- Fix for issue #255.
- matfaust Faust.double/single refactoring/adjustement in .m and mex and doc.
- New unit tests for matfaust FaustTest.testDouble/testSingle.