Add missing attributes to operators

Context

For some operators, not all ONNX supported attributes were added to Aidge. This MR is dedicated to complete those operators:

• Reshape: add 'allowzero' attribute
• Slice: add 'steps' input

Fixes a part of Issue_70 and Issue_10

Modified files

• Reshape.hpp, Reshape.cpp and pybind_Reshape.cpp, add 'allowzero' attribute to Reshape operator;
• Slice.hpp, Slice.cpp and pybind_Slice.cpp, add 'steps' attribute to Slice operator;

TODO

• Add all Reshape attributes
• Add all Slice attributes