Adding new operator Atan

Issue to track progress on integration of new operator Atan:

• Create Operator Atan Done in aidge_core!163 (1be1a5cc)
• Create Implementation Atan aidge_backend_cpu!92 (684f7bc6)
• Bind Atan to python aidge_core!163 (1be1a5cc)
• Add unit test on Atan implementation aidge_backend_cpu!92 (684f7bc6)
• Add Atan to ONNX import aidge_onnx!74 (780d413e)
• Add kernel for Atan in export arm cortexm