Moving access modes for data handles from struct starpu_task to struct starpu_codelet
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TODO list

- Implement test for checking old functionalities still work (similar to tests/core/deprecated)

- Make struct starpu_buffer_descr private (or not, as it can still be used in tests and examples)

- Set task->buffers to NULL

- Update all tests and examples

- When cost_model is provided, but not cost_function, need to rebuild a struct starpu_buffer_descr

- Update doc and changelog