clean up frame and representation treatment

[nstarman]

Description

It was pretty scattered. Now strings, classes, instances, None, and Ellipsis should be handled the same way for both frames and representations.

PR Checklist

• Check out the contributing guidelines and code of conduct
• Check out the contributing workflow ( for a practical example click here )
• Give a detailed description of the PR above.
• Document changes in the CHANGES.rst file. See existing changelog for examples.
• Add tests, if applicable, to ensure code coverage never decreases.
• Make sure the docs are up to date, if applicable, particularly the docstrings and RST files in docs folder.
• Ensure linear history by rebasing, when requested by the maintainer.

[codecov]

Codecov Report

Merging #45 (bc5487d) into master (f808c5a) will increase coverage by 1.37%.
The diff coverage is 89.41%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   83.87%   85.24%   +1.37%     
==========================================
  Files          25       26       +1     
  Lines        1234     1349     +115     
==========================================
+ Hits         1035     1150     +115     
  Misses        199      199              

Impacted Files	Coverage Δ
discO/core/core.py	100.00% <ø> (ø)
discO/plugin/agama/fitter.py	0.00% <0.00%> (ø)
discO/plugin/agama/sample.py	0.00% <0.00%> (ø)
discO/plugin/agama/wrapper.py	0.00% <0.00%> (ø)
discO/core/pipeline.py	64.37% <76.92%> (+16.40%)	⬆️
discO/core/wrapper.py	98.50% <98.50%> (ø)
discO/config.py	100.00% <100.00%> (ø)
discO/core/__init__.py	100.00% <100.00%> (ø)
discO/core/fitter.py	100.00% <100.00%> (ø)
discO/core/measurement.py	99.00% <100.00%> (+0.04%)	⬆️
... and 7 more

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