Fix CPP export for Add Sub and add export for Mul

Context

Add, Mul and Sub did not export correctly through aidge_export_cpp.

Fixed and add export for Mul

Modified files

Added in aidge_expot_cpp/operators.py;

• fixes in AddCPP and SubCPP class
• implementation of MulCPP class + registration of operator Mul

Detailed major modifications

No major modification, except the addition of the support for Mul export.

TODO

No todo forecasted.

Note though that the code for the element wise operators could be factorized out in the future.

Note also, that there is for now unit tests missing for operator exports, could contribute to this in the future.