Improve design consistency in the Prefect Workflow

[krlberry]

Improve design consistency:

• Update all flows to use a single context throughout the whole workflow
  • Prerequisite: Develop a consistent Context mechanism for the the Prefect workflow #28
• Merge more-or-less duplicated stages for import
  • calibration and target data import and prep stages call the same parent flow
  • Update the calibration pipeline stages to include the iterative solver loops used (Partially compeleted)
• Settle on a consistent “fake data” format (or formats) that all stages of the pipeline will support
  • same type, dimension keys on every data object returned by a stage flow
  • data object returned from simulated archive query task in "data import and prep" stages passed via context all the way to imaging stages
• Update all files to use the update fake data format:
  • stage_bandpass_solve.py
  • stage_calibrator_import_and_prep.py
  • stage_data_import_and_prep.py
  • stage_findcont.py
  • stage_image_calibrators.py
  • stage_image_cont_selfcal.py
  • stage_image_cube.py
  • stage_image_perspw_cont.py
  • stage_time_gain_solve.py

[krlberry]

All flows were updated to use a consistent context.

I've updated stage_time_gain_solve and stage_bandpass_solve to use the common data format, and attempt to fetch this from the context, and use the interactive solve(), but stage_image_calibrators still needs to be updated in this manner as well .