Merge branch 'user-guide' into 'master'

bob/pipelines/__init__.py

bob/pipelines/distributed/__init__.py

bob/pipelines/distributed/sge.py

bob/pipelines/mixins.py

bob/pipelines/sample.py

bob/pipelines/script/__init__.py

bob/pipelines/utils.py

conda/meta.yaml

doc/checkpoint.rst

doc/conf.py

doc/dask.rst

doc/extra-intersphinx.txt

doc/img/checkpoint.png

doc/img/dask.png

doc/img/metadata.png

doc/index.rst

doc/nitpick-exceptions.txt

doc/py_api.rst

doc/python/dask_graph.png

doc/python/pipeline_example.py