Merge branch 'modularise_dag_creation' into 'master'

bilby_pipe/job_creation/__init__.py

bilby_pipe/job_creation/bilby_pipe_dag_creator.py

bilby_pipe/job_creation/dag.py

bilby_pipe/job_creation/node.py

bilby_pipe/job_creation/nodes/__init__.py

bilby_pipe/job_creation/nodes/analysis_node.py

bilby_pipe/job_creation/nodes/generation_node.py

bilby_pipe/job_creation/nodes/merge_node.py

bilby_pipe/job_creation/nodes/pe_summary_node.py

bilby_pipe/job_creation/nodes/plot_node.py

bilby_pipe/job_creation/nodes/post_processing_node.py

bilby_pipe/job_creation/overview.py

bilby_pipe/job_creation/slurm.py

bilby_pipe/main.py

setup.py

tests/main_test.py