Choose backends

Created originally on Bitbucket by paugier (Pierre Augier)

Was already merged in Bitbucket before import, marked as merged by the import user

• More reasonable use of backend_default
• Solve path_jit issues
• Fix ipynb test + more backend choice
• Fix a bug with can_import_accelerator()
• Rewrite the comparison Numba / Pythran with Transonic!
• test_set_backend_for_this_module
• Doc: writing benchmarks
• Update doc packaging
• Fix an issue with Numba in setup.py

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