MaxPooling2D backward implementation for CPU backend

Context

Through this Merge-Request is illustrated the code for the backward cpu implementation of Maxpool2D operator (find related issue here).

Modified files

• include/aidge/backend/cpu/operator/MaxPoolingImpl.hpp, include/aidge/backend/cpu/operator/MaxPoolingImpl_kernels.hpp , src/operator/MaxPoolingImpl.cpp : add backward implementation for the operator
• unit_tests/operator/Test_MaxPoolingImpl.cpp : Add unit tests for MaxPool backward