WIP: Culler and Ender Future PSF Photometry API

[Onoddil]

This pull request is set up for the discussion of the forward-looking additions to PSF photometry and should not be merged until after the discussion is finished. In this case the issue is the 'culler and ender' aspect of the block diagram. This API documentation complements that in #721 where the documentation for those blocks implemented, with solid APIs unlikely to change in future releases. Please see #766 for an overview of the fitting process and a revised block diagram.

The structure of the blocks input and output parameters is described in detail in the documentation, but the main three stages are summarised below:

• CullerAndEnder
  • Formatting
  • Wording
  • API workflow: requires data (~astropy.table.Table), psf_model (~astropy.modeling.Fittable2DModel), and new_sources (~astropy.table.Table) and returns new_data (~astropy.table.Table) and end_flag (boolean).
• cull_data
  • Formatting
  • Wording
  • API workflow: requires data and psf_model and returns new_data (~astropy.table.Table)
• end_loop
  • Formatting
  • Wording
  • API workflow: requires new_data, data and new_sources, returning end_flag (boolean)

Please provide any feedback on the API for this PSF photometry fitting routine block you may have, such that the implementation meets all of the requirements of all users and is as clear as possible going forward. A simple example for this block, maintaining "do nothing" backwards-compatibility can be found here, with example implementation in IterativelySubtractedPSFPhotometry. Please also provide simpler formatting or grammatical changes to improve the readability and professional look of the document. The function's primary goals are to remove poor quality fits from the list of sources returned to the user, and break the iterative process if certain criteria are met.