Add first export tests

Context

Linked with #66 (closed)

Add unit test for the conversion Aidge -> ONNX

Changes

• dev: Add test to check conversion aidge -> onnx (check onnx is valid and then check the generated ONNX give the same results as the origianl one)
• fix: Update use of is_optional_input function to match new signature
• feat: Add possibility to set the IR version when generating an ONNX
• dev: Remove from the xfail list various tests that were passing