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)

- Update all tests and examples

- Depreciate cost_model field in struct starpu_per_arch_perfmodel
                                 struct starpu_perfmodel

- Add a field cost_model_new
  double (*cost_model_new)(starpu_data_handle_t *); /* returns expected duration in µs */

- Update all cost functions in examples

- Set task->buffers to NULL

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

- Update doc and changelog