Remove unused transformers, rename is_instance_nested, fix API docs

bob/pipelines/__init__.py

bob/pipelines/tests/test_transformers.py

bob/pipelines/tests/test_utils.py

bob/pipelines/transformers/__init__.py

bob/pipelines/transformers/file_loader.py

bob/pipelines/transformers/linearize.py

bob/pipelines/transformers/pca.py

bob/pipelines/utils.py

bob/pipelines/wrappers.py

bob/pipelines/xarray.py

doc/py_api.rst