Refactor: unnest pipeline nodes

[liyier90]

Closes https://github.com/aisingapore/PeekingDuck-Private/issues/98

Removes the pipeline folder.

• peekingduck/pipeline/nodes is moved to peekingduck/nodes
• peekingduck/pipeline/utils is moved to peekingduck/utils
• peekingduck/pipeline/pipeline.py is moved to peekingduck/pipeline.py

Updated import statements, tutorials and tests to reflect the change.

[ongtw]

LGTM: CI/CD passed on M1 Mac local run and documentation all looks updated.