modularise job creation code

• bilby_pipe.main is getting very large (>1000 lines of code) and has a lot going on
• this MR moves the condor/slurm job creation code into a separate module

If the above is considered a good idea, I am also happy to add docs to the new files that have been created.

bilby_pipe/job_creation/nodes/__init__.py

+from .analysis_node import AnalysisNode
+from .generation_node import GenerationNode
+from .merge_node import MergeNode
+from .pe_summary_node import PESummaryNode
+from .plot_node import PlotNode
+from .post_processing_node import (
+    PostProcessAllResultsNode,
+    PostProcessSingleResultsNode,
+)