[MRG+1] Change named_steps to Bunch object (#8586)

* Change named_steps to Bunch object

* Update named_steps attribute documentation

* Add test for named steps bunch object

* Delete whitespace in test_pipeline

* Update test_pipeline.py

* Add comment for named_steps usage

* Move dataset/Bunch to utils

* Fix to PEP8 format

* Add __getattribute method to Bunch class, Fix pep8 bug

* Remove __getattribute__, update test_pipeline

* Update test with conflict and non-conflict named_steps

* Add reference to class Pipeline