Transition from working group metapackages to pipeline metapackages

To more effectively capture what is needed on a given system, I think we should transition away from the current working group metapackages (e.g. lscsoft-external-cbc) to pipeline-based metapackages (e.g. igwn-cwb, igwn-xpipeline) that include only those system requirements for that pipeline.

I think this will be easier to maintain and keep up-to-date, especially when new pipelines are added or pipelines are discontinued.

cc @adam-mercer @stuart.anderson @pfc @steffen.grunewald

Pipeline metapackage to-do list:

• Burst:
  • cWB
  • STAMP
  • X-Pipeline [#41]
• CBC:
  • Bilby
  • GstLAL
  • LALInference
  • MBTA | Virgo
  • PyCBC | no requirements (confirmed with Harry)
  • SPIIR
  • Tiger
• CW:
  • [ ]
  • TD Bayesian | no requirements (confirmed with Pitkin)
• Low-latency
  • gwcelery [#42]
• Stochastic:
  • [ ]
  • [ ]