Add GPU2 wrapper in matfaust for Faust functions: wht, dft, eye, isreal....

Add GPU2 wrapper in matfaust for Faust functions: wht, dft, eye, isreal. Simplify mex generation (only one source file for both CPU and GPU2 wrappers).

- isReal() function moved up in TransformHelperGen parent.
- Fix of set_Fv/FM_mul_mode signatures of GPU2 TransformHelper.
- Update of CMakeLists.txt for mexFaust wrapper generation (one unique mex file using generic programming for Cpu and GPU2).