Merge branch 'refactor-csv-database' into 'master'

bob/pipelines/config/distributed/dask_it_pipeline.py

bob/pipelines/datasets.py

bob/pipelines/sample_loaders.py

bob/pipelines/tests/data/samples/samples.csv

bob/pipelines/tests/test_datasets.py

bob/pipelines/tests/test_sample_loaders.py

bob/pipelines/tests/test_transformers.py

bob/pipelines/tests/test_utils.py

bob/pipelines/tests/test_wrappers.py

bob/pipelines/tests/test_xarray.py

bob/pipelines/wrappers.py

doc/checkpoint.rst

doc/dask.rst

doc/datasets.rst

doc/sample.rst

doc/xarray.rst