Fix padding

Fix for aidge#194 (closed).

According to ONNX: pads format should be: [x1_begin, x2_begin, …, x1_end, x2_end,…], where xi_begin is the number of pad values added at the beginning of axis axes[i] and xi_end, the number of pad values added at the end of axis axes[i].

Fixed issues:

• The dimensions were mixed up in oxSize and oySize, which assumed the wrong [x1_begin, x1_end, …, x2_begin, x2_end,…] format.
• begin and end padding was mixed up in the computation of ix and iy.
• The axis order was wrong in the unit test.